xen: improve shutdown hook

Make better use of the shutdown flags. In particular this now handles
standard reboot where RB_POWERCYCLE is not set, and indicates a crash
when the system has panicked.

While here, give the function a prefix.

Reviewed by: royger, markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42343