secret: Add check/validation for correct usage when LookupByUUID

https://bugzilla.redhat.com/show_bug.cgi?id=1656255

If virSecretGetSecretString is using by secretLookupByUUID,
then it's possible the found sec->usageType doesn't match the
desired @secretUsageType. If this occurs for the encrypted
volume creation processing and a subsequent pool refresh is
executed, then the secret used to create the volume will not
be found by the storageBackendLoadDefaultSecrets which expects
to find secrets by VIR_SECRET_USAGE_TYPE_VOLUME.

Add a check to virSecretGetSecretString to avoid the possibility
along with an error indicating the incorrect matched types.

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>