tests: Adjust LUKS tests to use 'volume' secret type

tests: Adjust LUKS tests to use 'volume' secret type

Commit id's '9bbf0d7e6' and '2552fec24' added some XML parsing tests
for a LUKS volume to use a 'passphrase' secret format. After commit,
this was deemed to be incorrect, so covert the various tests to use
the volume usage format where the 'usage' is the path to the volume
rather than a user defined name string.

Also, removed the qemuxml2argv-luks-disk-cipher.xml since it was
just a duplicate of qemuxml2argv-luks-disks.xml.

Signed-off-by: John Ferlan <jferlan@redhat.com>