Attach encryption information to virStorageVolDef.

The XML allows <encryption format='unencrypted'/>, this implementation
canonicalizes the internal representation so that "vol->encryption" is
non-NULL iff the volume is encrypted.

Note that partial encryption information (e.g. specifying an encryption
format, but not the key/passphrase) is valid, libvirt will automatically
choose value for the missing information during volume creation.  The
user can read the volume XML, and use the unmodified <encryption> tag in
future operations (without having to be able to understand) its contents.

* docs/formatstorage.html, docs/formatstorage.html.in: Document
  storage volume encryption options
* src/storage_conf.c, src/storage_conf.h: Hook up storage
  encryption XML handling
* tests/storagevolschemadata/vol-qcow2.xml: Test case for encryption
  schema changes

tests/storagevolschemadata/vol-qcow2.xml

@@ -14,6 +14,9 @@
       <group>0</group>
       <label>unconfined_u:object_r:virt_image_t:s0</label>
     </permissions>
+    <encryption format='qcow'>
+      <secret type='passphrase' uuid='e78d4b51-a2af-485f-b0f5-afca709a80f4'/>
+    </encryption>
   </target>
   <backingStore>
     <path>/var/lib/libvirt/images/BaseDemo.img</path>