bhyve: Move the vm_inject_fault() implementation to vmexit.c

This function isn't generic and has a different signature on arm64. No
functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40991

(cherry picked from commit 7228ad8da93bcce820c85b733972d0d9cc767d27)