hwrng: virtio - unregister device before reset

unregister after reset is clearly wrong - device
can be used while it's reset. There's an attempt to
protect against that using hwrng_removed but it
seems racy since access can be in progress
when the flag is set.

Just unregister, then reset seems simpler and cleaner.
NB: we might be able to drop hwrng_removed in a follow-up patch.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>