luks: Create block_crypto_co_create_generic()

luks: Create block_crypto_co_create_generic()

Everything that refers to the protocol layer or QemuOpts is moved out of
block_crypto_create_generic(), so that the remaining function is
suitable to be called by a .bdrv_co_create implementation.

LUKS is the only driver that actually implements the old interface, and
we don't intend to use it in any new drivers, so put the moved out code
directly into a LUKS function rather than creating a generic
intermediate one.

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