qcow2: Use QCryptoBlockCreateOptions in qcow2_co_create()

Instead of passing the encryption format name and the QemuOpts down, use
the QCryptoBlockCreateOptions contained in BlockdevCreateOptions.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>