monitor: Make MonitorQMP a child class of Monitor

monitor: Make MonitorQMP a child class of Monitor

Currently, struct Monitor mixes state that is only relevant for HMP,
state that is only relevant for QMP, and some actually shared state.
In particular, a MonitorQMP field is present in the state of any
monitor, even if it's not a QMP monitor and therefore doesn't use the
state.

As a first step towards a clean separation between QMP and HMP, let
MonitorQMP extend Monitor and create a MonitorQMP object only when the
monitor is actually a QMP monitor.

Some places accessed Monitor.qmp unconditionally, even for HMP monitors.
They can't keep doing this now, so during the conversion, they are
either changed to become conditional on monitor_is_qmp() or to assert()
that they always get a QMP monitor.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190613153405.24769-4-kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Superfluous variable in monitor_data_destroy() eliminated]
Signed-off-by: Markus Armbruster <armbru@redhat.com>