qapi: fix leak in unit tests

qmp_output_get_qobject() increments the qobject's reference count. Since
we currently pass this straight into qobject_to_json() so we can feed
the data into a QMP input visitor, we never actually free the underlying
qobject when qmp_output_visitor_cleanup() is called. This causes leaks
on all of the QMP serialization tests.

Fix this by holding a pointer to the qobject and decref'ing it before
returning from qmp_deserialize().

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
