virttest: Record the output from QMP monitor when failed to get the greeting message

Record the json output from QMP monitor when we raise the MonitorProtocolError. It
may help us to analyse what failed this step.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1078 from ypu/qmp

virttest: Record the output from QMP monitor when failed to get the gree...