The current registration of the Xen specific shutdown handler is tied to
xenstore, as it's registered by a device that depends on xenstore being
available. However the shutdown hypercalls don't need xenstore, and hence
registration must not be tied to xenstore (as some domains might not even
have xenstore).

Move the registration of the shutdown handler into it's own isolated
function that gets called directly from a SYSINIT() block. Place the code
in xen_common.c, since the shutdown hypercall is available in all arches.