x86/xen: select HIBERNATE_CALLBACKS more directly

The config XEN_SAVE_RESTORE's only purpose is to select
HIBERNATE_CALLBACKS, when config XEN is set. The XEN config definition can
simply select HIBERNATE_CALLBACKS, though, and the definition of
XEN_SAVE_RESTORE can be dropped.

So, remove this indirection through XEN_SAVE_RESTORE and select
HIBERNATE_CALLBACKS directly. Also, drop the XEN_SAVE_RESTORE from the x86
xen config fragment.

No functional change intended with this clean-up.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250829070402.159390-1-lukas.bulwahn@redhat.com>