Fix the filename of debian/xen-hypervisor-V-F.postrm by adding the
missing .vsn-in suffix so it actually gets installed.
This fixes the issue of not running update-grub when the Xen hypervisor
packages get removed, which resulted in a system that could not be
rebooted without interactive usage of the grub menu.
Signed-off-by: Maximilian Engelhardt <maxi@daemonizer.de> Reviewed-by: Hans van Kranenburg <hans@knorrie.org>