util: Add 'usage' for encryption

util: Add 'usage' for encryption

In order to use more common code and set up for a future type, modify the
encryption secret to allow the "usage" attribute or the "uuid" attribute
to define the secret. The "usage" in the case of a volume secret would be
the path to the volume as dictated by the backwards compatibility brought
on by virStorageGenerateQcowEncryption where it set up the usage field as
the vol->target.path and didn't allow someone to provide it. This carries
into virSecretObjListFindByUsageLocked which takes the secret usage attribute
value from from the domain disk definition and compares it against the
usage type from the secret definition. Since none of the code dealing
with qcow/qcow2 encryption secrets uses usage for lookup, it's a mostly
cosmetic change. The real usage comes in a future path where the encryption
is expanded to be a luks volume and the secret will allow definition of
the usage field.

This code will make use of the virSecretLookup{Parse|Format}Secret common code.

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