xend: NetBSD portability fix for LVM raw volume names

Xen 4.1.1 was incorrectly passing /dev/rmapper/vg-lvname to pygrub
(notice the r in front of mapper), when it should pass
/dev/mapper/rvg-lvname (add the r to the last file) when using NetBSD.
I've patched it to work correctly. I'm attaching a unified diff with
the patch made against Xen 4.1.1 (it's a really simple modification).

From: Roger Pau Monne <roger.pau@entel.upc.edu>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>
xen-unstable changeset:   23629:89ce3439686b
xen-unstable date:        Tue Jun 28 13:56:53 2011 +0100

tools/python/xen/xend/XendBootloader.py

@@ -44,7 +44,7 @@ def bootloader(blexec, disk, dom, quiet = False, blargs = '', kernel = '',
         raise VmError(msg)
 
     if os.uname()[0] == "NetBSD" and disk.startswith('/dev/'):
-       disk = disk.replace("/dev/", "/dev/r")
+       disk = "/r".join(disk.rsplit("/",1))
 
     mkdir.parents("/var/run/xend/boot/", stat.S_IRWXU)