hw/arm: Do not expose the virt machine on Xen-only binary

hw/arm: Do not expose the virt machine on Xen-only binary

Since the Virt machine is useless under Xen, do not even
try to build it there.
A Xen-only binary now only offers the XenPVH machine:

  $ qemu-system-aarch64 -M help
  Supported machines are:
  none                 empty machine
  xenpvh               Xen PVH ARM machine

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Message-Id: <20250218162618.46167-3-philmd@linaro.org>