For better code sharing, add a helper function that handles
reference counting of the virtio backend for virtio proxy devices.

Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>