storage: Add support to create a luks volume

storage: Add support to create a luks volume

Partially resolves:
https://bugzilla.redhat.com/show_bug.cgi?id=1301021

If the volume xml was looking to create a luks volume take the necessary
steps in order to make that happen.

The processing will be:
 1. create a temporary file (virStorageBackendCreateQemuImgSecretPath)
   1a. use the storage driver state dir path that uses the pool and
       volume name as a base.

 2. create a secret object (virStorageBackendCreateQemuImgSecretObject)
   2a. use an alias combinding the volume name and "_luks0"
   2b. add the file to the object

 3. create/add luks options to the commandline (virQEMUBuildLuksOpts)
   3a. at the very least a "key-secret=%s" using the secret object alias
   3b. if found in the XML the various "cipher" and "ivgen" options

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