monitor: send event when command queue full

monitor: send event when command queue full

Set maximum QMP command queue length to 8.  If the queue is full,
instead of queuing the command, we directly return a "command-dropped"
event, telling the client that a specific command is dropped.

Note that this flow control mechanism is only valid if OOB is enabled.
If it's not, the effective queue length will always be 1, which strictly
follows original behavior of QMP command handling (which never drops
messages).

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-17-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: commit message grammar, abort on failure to send event]
Signed-off-by: Eric Blake <eblake@redhat.com>