libqtest: Ignore QMP events when parsing the response for HMP commands

When running certain HMP commands (like "device_del") via QMP, we
can sometimes get a QMP event in the response first, so that the
"g_assert(ret)" statement in qtest_hmp() triggers and the test
fails. Fix this by ignoring such QMP events while looking for the
real return value from QMP.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1490860207-8302-2-git-send-email-thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Added note to qtest_hmp/qtest_hmpv's header description to say
  it discards events