tests/qtest: add support for callback to receive QMP events

tests/qtest: add support for callback to receive QMP events

Currently code must call one of the qtest_qmp_event* functions to
fetch events. These are only usable if the immediate caller knows
the particular event they want to capture, and are only interested
in one specific event type. Adding ability to register an event
callback lets the caller capture a range of events over any period
of time.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230601161347.1803440-3-berrange@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>