conf: add encryption engine property

This commit extends libvirt XML configuration to support a custom encryption engine.
This means that <encryption format="luks" engine="qemu">  becomes valid.
The only engine for now is qemu. However, a new engine (librbd) will be added in an upcoming commit.
If no engine is specified, qemu will be used (assuming qemu driver is used).

Signed-off-by: Or Ozeri <oro@il.ibm.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

docs/formatstorageencryption.html.in

@@ -23,6 +23,12 @@
       content of the <code>encryption</code> tag.  Other format values may be
       defined in the future.
     </p>
+    <p>
+      The <code>encryption</code> tag supports an optional <code>engine</code>
+      tag, which allows selecting which component actually handles
+      the encryption. Currently defined values of <code>engine</code> are
+      <code>qemu</code>.
+    </p>
     <p>
       The <code>encryption</code> tag can currently contain a sequence of
       <code>secret</code> tags, each with mandatory attributes <code>type</code>

docs/schemas/domainbackup.rng

@@ -14,6 +14,13 @@
           <value>luks</value>
         </choice>
       </attribute>
+      <optional>
+        <attribute name="engine">
+          <choice>
+            <value>qemu</value>
+          </choice>
+        </attribute>
+      </optional>
       <interleave>
         <ref name="secret"/>
         <optional>

docs/schemas/storagecommon.rng

@@ -15,6 +15,13 @@
           <value>luks</value>
         </choice>
       </attribute>
+      <optional>
+        <attribute name="engine">
+          <choice>
+            <value>qemu</value>
+          </choice>
+        </attribute>
+      </optional>
       <interleave>
         <ref name="secret"/>
         <optional>