monitor: Turn monitor_qapi_event_state[] into a hash table

In preparation of finer grained throttling.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1444921716-9511-6-git-send-email-armbru@redhat.com>