x86/xen: add some __init and static annotations in arch/x86/xen/setup.c

Some more functions in arch/x86/xen/setup.c can be made "__init".
xen_ignore_unusable() can be made "static".

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>