crypto: virtio - Register an algo only if it's supported

Register a crypto algo with the Linux crypto layer only if
the algorithm is supported by the backend virtio-crypto
device.

Also route crypto requests to a virtio-crypto
device, only if it can support the requested service and
algorithm.

Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Acked-by: Gonglei <arei.gonglei@huawei.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>