vmx: Implement pmap (de)activation in C

Rewrite the code that maintains pm_active and invalidates EPTP-tagged
TLB entries in C. Previously this work was done in vmx_enter_guest(),
in assembly, but there is no good reason for that and it makes the TLB
invalidation algorithm for nested page tables harder to review.

No functional change intended. Now, an error from the invept
instruction results in a kernel panic rather than a vmexit. Such errors
should occur only as a result of VMM bugs.

Reviewed by: grehan, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26830