iotests/149: Skip on unsupported ciphers

iotests/149: Skip on unsupported ciphers

Whenever qemu-img or qemu-io report that some cipher is unsupported,
skip the whole test, because that is probably because qemu has been
configured with the gnutls crypto backend.

We could taylor the algorithm list to what gnutls supports, but this is
a test that is run rather rarely anyway (because it requires
password-less sudo), and so it seems better and easier to skip it.  When
this test is intentionally run to check LUKS compatibility, it seems
better not to limit the algorithms but keep the list extensive.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211117151707.52549-3-hreitz@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>