qemu: Add support for controling qemu capabilities via the qemu XML namespace

Similarly how we allow adding arbitrary command line arguments and
environment variables this patch introduces the ability to control
libvirt's perception of the qemu process by tweaking the capability bits
for testing purposes.

The idea is to allow developers and users either test a new feature by
enabling it early or disabling it to see whether it introduced
regressions.

This feature is not meant for production use though, so users should
handle it with care.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

docs/drvqemu.html.in

@@ -519,6 +519,36 @@ mount -t cgroup none /dev/cgroup -o devices
     <qemu:env name='QEMU_ENV' value='VAL'/>
   </qemu:commandline>
 </domain>
+</pre>
+
+    <h2><a id="xmlnsfeatures">QEMU feature configuration for testing</a></h2>
+
+      <p>
+        In some cases e.g. when developing a new feature or for testing it may
+        be required to control a given qemu feature (or qemu capability) to test
+        it before it's complete or disable it for debugging purposes.
+        <span class="since">Since 5.5.0</span> it's possible to use the same
+        special qemu namespace as above
+        (<code>http://libvirt.org/schemas/domain/qemu/1.0</code>) and use
+        <code>&lt;qemu:capabilities&gt;</code> element to add
+        (<code>&lt;qemu:add capability="capname"/&gt;</code>) or remove
+        (<code>&lt;qemu:del capability="capname"/&gt;</code>) capability bits.
+        The naming of the feature bits is the same libvirt uses in the status
+        XML. Note that this feature is meant for experiments only and should
+        _not_ be used in production.
+      </p>
+
+      <p>Example:</p><pre>
+&lt;domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'&gt;
+  &lt;name&gt;testvm&lt;/name&gt;
+
+   [...]
+
+  &lt;qemu:capabilities&gt;
+    &lt;qemu:add capability='blockdev'/&gt;
+    &lt;qemu:del capability='drive'/&gt;
+  &lt;/qemu:capabilities&gt;
+&lt;/domain&gt;
 </pre>
 
     <h2><a id="xmlconfig">Example domain XML config</a></h2>

docs/schemas/domaincommon.rng

@@ -72,6 +72,9 @@
         <optional>
           <ref name='qemucmdline'/>
         </optional>
+        <optional>
+          <ref name='qemucapabilities'/>
+        </optional>
         <optional>
           <ref name='lxcsharens'/>
         </optional>
@@ -6197,6 +6200,22 @@
     </element>
   </define>
 
+  <define name="qemucapabilities">
+    <element name="capabilities" ns="http://libvirt.org/schemas/domain/qemu/1.0">
+      <zeroOrMore>
+        <element name="add">
+          <attribute name="capability"/>
+        </element>
+      </zeroOrMore>
+      <zeroOrMore>
+        <element name="del">
+          <attribute name="capability"/>
+        </element>
+      </zeroOrMore>
+    </element>
+  </define>
+
+
   <!--
        Optional hypervisor extensions in their own namespace:
        LXC