Move grub config into xen-hypervisor-common

Move grub config into xen-hypervisor-common

The grub config snippet is not arch dependent, neither version
dependent, so it has to move into a 'common' package.

The packages with a version number in their name only have contents that
do not conflict with a previous or next version. Having this file in the
version dependent xen-hypervisor package currently prevents us from
being able to co-install the current and next version of Xen.

Since users who install the whole set of hypervisor and utils via the
meta package undoubtedly also want to get the example grub config,
always install the xen-hypervisor-common package in that case, even if
the user ignores Recommends (like I always do).

Also see #852545

Signed-off-by: Hans van Kranenburg <hans@knorrie.org>