monitor: let suspend/resume work even with QMPs

This patches allows QMP monitors to be suspended/resumed.

One thing to mention is that for QMPs that are using IOThreads, we need
an explicit kick for the IOThread in case it is sleeping.

Meanwhile, we need to take special care on non-interactive HMPs.
Currently only gdbserver is using that.  For these monitors, we still
don't allow suspend/resume operations.

Since at it, add traces for the operations.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-14-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>