hyperv: register intr handler as usermode-mapped if loaded as module

Normally raw interrupt handler is provided by the kernel text. But
vmbus module registers its own handler that needs to be mapped into
userspace mapping on PTI kernels.

Reported and reviewed by: whu
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30310