x86/xen: cleanup arch/x86/xen/setup.c

Remove extern declarations in arch/x86/xen/setup.c which are either
not used or redundant. Move needed other extern declarations to
xen-ops.h

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