libxenvchan: Change license of header from Lesser GPL v2.1 to BSD

As the xen/COPYING file says:
"A few files are licensed under both GPL and a weaker BSD-style
license. This includes all files within the subdirectory
include/public, as described in include/public/COPYING. All such files
include the non-GPL license text as a source-code comment. Although
the license text refers generically to "the software", the non-GPL
license applies *only* to those source files that explicitly include
the non-GPL license text."

The libxenvchan.h is under xen/include/public/io directory
and the xen/include/public/COPYING says:

"XEN NOTICE
==========

This copyright applies to all files within this subdirectory and its
subdirectories:
  include/public/*.h
  include/public/hvm/*.h
  include/public/io/*.h

The intention is that these files can be freely copied into the source
tree of an operating system when porting that OS to run on Xen. Doing
so does *not* cause the OS to become subject to the terms of the GPL.

All other files in the Xen source distribution are covered by version
2 of the GNU General Public License except where explicitly stated
otherwise within individual source files.
"
Having the libxenvchan.h as Lesser GPL v2.1 where the COPYING file
says otherwise is confusing to say at least.

Upon consulting with the authors of libxenvchan they said:
"FWIW Neither I, nor ITL staff (as author of original libvchan library)
have anything against converting it to the BSD-style licence."
(Marek Marczykowski-Górecki,
http://lists.xen.org/archives/html/xen-devel/2016-06/msg00995.html)
so as such lets change it.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Anil Madhavapeddy <anil@recoil.org>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: George Dunlap <George.Dunlap@eu.citrix.com>
Acked-by: Jan Beulich <JBeulich@suse.com>
Acked-by: Jason Andryuk <andryuk@aero.org>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Matthew Daley <mattjd@gmail.com>
Acked-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
["I have spoken to my line manager.  I can confirm that Citrix is happy
 with this proposed change.  So:

 Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

 This view from Citrix covers all contributions made to these files in
 the course of Citrix's employees' employment, which I think is:

 > Cc: Andrew Cooper <andrew.cooper3@citrix.com>
 > cc: George Dunlap <George.Dunlap@eu.citrix.com>
 > Cc: Ian Campbell <ian.campbell@citrix.com>
 > Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
 > Cc: Roger Pau Monne <roger.pau@entel.upc.edu>
 > Cc: Stefano Stabellini <sstabellini@kernel.org>
 > Cc: Tim Deegan <tim@xen.org>
 > Cc: Wei Liu <wei.liu2@citrix.com>

 ..
 [in subsequent email]:
 Wei points out that this ought also to include Keir Fraser's
 contribution, which was (only) in 2012.
 " (from Ian's email)

 In a subsequent mail, Wei also points out that David Scott's
 contribution is covered by Ian's ack.
]

xen/include/public/io/libxenvchan.h

@@ -10,18 +10,23 @@
  *
  * @section LICENSE
  *
- *  This library is free software; you can redistribute it and/or
- *  modify it under the terms of the GNU Lesser General Public
- *  License as published by the Free Software Foundation; either
- *  version 2.1 of the License, or (at your option) any later version.
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to
+ * deal in the Software without restriction, including without limitation the
+ * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ * sell copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
  *
- *  This library is distributed in the hope that it will be useful,
- *  but WITHOUT ANY WARRANTY; without even the implied warranty of
- *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
- *  Lesser General Public License for more details.
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
  *
- *  You should have received a copy of the GNU Lesser General Public
- *  License along with this library; If not, see <http://www.gnu.org/licenses/>.
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
  *
  * @section DESCRIPTION
  *