net/virtio: fix re-plugging of primary device

failover_replug_primary was returning true on failure which lead to
re-plug not working when a migration failed.  Fix this by returning
success when hotplug worked.  This is a bug that was missed in last
round of testing but was tested succesfully with this version.  Also
make sure we don't pass NULL to qdev_set_parent_bus().

This fixes CID 1407224.

Fixes: 9711cd0d ("net/virtio: add failover support")
Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>