tests/qtest: replace qmp_discard_response with qtest_qmp_assert_success

tests/qtest: replace qmp_discard_response with qtest_qmp_assert_success

The qmp_discard_response method simply ignores the result of the QMP
command, merely unref'ing the object. This is a bad idea for tests
as it leaves no trace if the QMP command unexpectedly failed. The
qtest_qmp_assert_success method will validate that the QMP command
returned without error, and if errors occur, it will print a message
on the console aiding debugging.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230421171411.566300-2-berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>