/dev/hwrng is only functional if virtio-rng is working right, so let's
add a sanity check for this device node.

Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Tested-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20201215183623.110128-3-thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>