luks: Separate image file creation from formatting

luks: Separate image file creation from formatting

The crypto driver used to create the image file in a callback from the
crypto subsystem. If we want to implement .bdrv_co_create, this needs to
go away because that callback will get a reference to an already
existing block node.

Move the image file creation to block_crypto_create_generic().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>