tools/xenpvboot: remove as unable to convert to Python 3

The script heavily relies on the urlgrabber python module, which doesn't seem
to be packaged by all distros; it's missing from newer Debian versions at
least.

Also the usage of the commands module has been deprecated since Python 2.6, and
removed in Python 3, so the code would need to be re-written to not rely on
urlgrabber and the commands modules.

Arguably the purpose of the xenpvnetboot bootloader can also be achieved with
an isolated script that fetches the kernel and ramdisk before attempting to
launch the domain, without having to run in libxl context.  There are no xl.cfg
options consumed by the bootloader apart from the base location and the
subpaths of the kernel and ramdisk to fetch.

Any interested parties in keeping such functionality are free to submit an
updated script that works with Python 3.

Resolves: xen-project/xen#172
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

CHANGELOG.md

@@ -38,6 +38,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
  - On x86, the "pku" command line option has been removed.  It has never
    behaved precisely as described, and was redundant with the unsupported
    "cpuid=no-pku".  Visibility of PKU to guests should be via its vm.cfg file.
+ - xenpvnetboot removed as unable to convert to Python 3.
 
 ## [4.17.0](https://xenbits.xen.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.17.0) - 2022-12-12