virtio-rng: Fix crash with non-default backend

'default_backend' isn't always set, but 'rng' is, so use that.

$ ./x86_64-softmmu/qemu-system-x86_64 -object rng-random,id=rng0,filename=/dev/random -device virtio-rng-pci,rng=rng0
Segmentation fault (core dumped)

Regressed with virtio refactoring in 59ccd20a9ac719cff82180429458728f03ec612f

CC: qemu-stable@nongnu.org
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Acked-by: Amit Shah <amit.shah@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Message-id: bf4505014a0a941dbd3c62068f3cf2c496b69e6a.1370023944.git.crobinso@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
