virtio-rng: Move error-checking forward to prevent memory leak

This patch pushes the error-checking forward and the virtio
initialization backward in the device realization function
in order to prevent memory leaks for hot plug scenarios.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
