qtest: Reintroduce qtest_qmp_receive with QMP event buffering

qtest: Reintroduce qtest_qmp_receive with QMP event buffering

The new qtest_qmp_receive buffers all the received qmp events, allowing
qtest_qmp_eventwait_ref to return them.

This is intended to solve the race in regard to ordering of qmp events
vs qmp responses, as soon as the callers start using the new interface.

In addition to that, define qtest_qmp_event_ref a function which only scans
the buffer that qtest_qmp_receive stores the events to.  This is intended
for callers that are only interested in events that were received during
the last call to the qtest_qmp_receive.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20201006123904.610658-3-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>