crypto: add support for the twofish cipher algorithm

New cipher algorithms 'twofish-128', 'twofish-192' and
'twofish-256' are defined for the Twofish algorithm.
The gcrypt backend does not support 'twofish-192'.

The nettle and gcrypt cipher backends are updated to
support the new cipher and a test vector added to the
cipher test suite. The new algorithm is enabled in the
LUKS block encryption driver.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>