bhyve: Register new kevents synchronously.

[jhb]

Summary

Change mevent_add*() to synchronously add the new kevent. This
permits reporting event registration failures to the caller and avoids
failing the registration of other, unrelated events queued up in the
same batch.

[markj]

usr.sbin/bhyve/mevent.c

465

Perhaps assert that no error occurred, or cast the return value away.

[jhb]

usr.sbin/bhyve/mevent.c

465

bhyve is pretty consistent about assuming no errors from mutexes, condvars, and once and not using either void casts or assertions of that (e.g. there's a once in block_if.c), so I think this is consistent with the rest of bhyve.