bhyve: Register new kevents synchronously.

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.

Reviewed by: grehan, markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D30502

(cherry picked from commit e8424e2947dd8a0e1d3f08295432941b5c5aafb9)