tests/qtest: add some tests for virtio-net failover

Add test cases to test several error cases that must be
generated by invalid failover configuration.

Add a combination of coldplug and hotplug test cases to be
sure the primary is correctly managed according the
presence or not of the STANDBY feature.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20211208130350.10178-3-lvivier@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>