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

(cherry picked from commit 9e0b0f5de67fd46bddf0e12ef7b71d76a7ec1667)