virtio-rng: Keep the default backend out of VirtIORNGConf

The default backend is only used within virtio_rng_device_realize().
Replace VirtIORNGConf member default_backend by a local variable.
Adjust its type to reduce conversions.

While there, pass &error_abort instead of NULL when failure would be a
programming error.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20190820160615.14616-3-lvivier@redhat.com>