virtio: link the rng backend through an alias property

The virtio-rng backend is currently linked twice, once in the proxy
device (e.g. virtio-rng-pci) and once in virtio-rng-device.  This causes
a double unref of the backend when the parent device is unplugged.

To fix this, make the proxy device use an alias, similar to what is
already being done for the iothread link.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Message-id: 1414577839-18695-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>