virtio-net: add missing object_unref()

virtio-net: add missing object_unref()

failover_add_primary() calls qdev_device_add() and doesn't unref
the device. Because of that, when the device is unplugged a reference
is remaining and prevents the cleanup of the object.

This prevents to be able to plugin back the failover primary device,
with errors like:

  (qemu) device_add vfio-pci,host=0000:41:00.0,id=hostdev0,bus=root.3,failover_pair_id=net0
  (qemu) device_del hostdev0

We can check with "info qtree" and "info pci" that the device has been removed, and then:

  (qemu) device_add vfio-pci,host=0000:41:00.0,id=hostdev1,bus=root.3,failover_pair_id=net0
  Error: vfio 0000:41:00.0: device is already attached
  (qemu) device_add vfio-pci,host=0000:41:00.0,id=hostdev0,bus=root.3,failover_pair_id=net0
  qemu-kvm: Duplicate ID 'hostdev0' for device

Fixes: 21e8709b29cd ("failover: Remove primary_dev member")
Cc: quintela@redhat.com
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20210212135250.2738750-3-lvivier@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>