Supports only virStorageVolCreateXML, not virStorageVolCreateXMLFrom.

Curiously, qemu-img does not need the passphrase for anything to create
an encrypted volume.  This implementation thus does not need to touch
any secrets to work with cooperating clients.  More generic passphrase
handling is added in the next patch.

* src/storage_backend.c: Request encryption when creating qcow/qcow2
  files
* src/storage_backend_disk.c, src/storage_backend_fs.c,
  src/storage_backend_logical.c: Refuse to create volumes with
  encryption params set.