libqtest: add qmp(fmt, ...) -> QDict* function

libqtest: add qmp(fmt, ...) -> QDict* function

Add a qtest qmp() function that returns the response object.  This
allows test cases to verify the result or to check for error responses.
It also allows waiting for QMP events.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Andreas Färber <[email protected]>