xen/pvh: Use Xen's emergency_restart op for PVH guests

Using native_machine_emergency_restart (called during reboot) will
lead PVH guests to machine_real_restart()  where we try to use
real_mode_header which is not initialized.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>