xen/x86: set panic notifier priority to minimum

Execution is not going to continue after telling Xen about the crash.
Let other panic notifiers run by postponing the final hypercall as much
as possible.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>