bhyve: Add support for EVFILT_VNODE mevents.

This allows registering an event to watch for changes to a file's
attributes. This is a bit imperfect as it would be nice to have a way
to determine if an fd can use EVFILT_VNODE successfully. mevent's
current structure does not permit that and a failure to register a
single kevent impacts several other kevents.

Reviewed by: grehan, markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D30503