conf: add support for VNC power control setting

The <graphics type="vnc" .... powerControl="yes"/> option instructs the
VNC server to enable an extension that lets the client perform a
graceful shutdown, reboot and hard reset.

This is enabled by default since it cannot be assumed that the VNC
client user has administrator rights over the guest OS. In the case
where the VNC user is a guest administrator though, it is reasonable
to allow direct power control host side too.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/formatdomain.rst

@@ -5791,6 +5791,11 @@ interaction with the admin.
       ``autoport`` having no effect due to security reasons) :since:`Since
       1.0.6` .
 
+      For VNC, the ``powerControl`` attribute can be used to enable VM shutdown,
+      reboot and reset power control features for the VNC client. This is
+      appropriate if the authenticated VNC client user already has administrator
+      privileges in the guest :since:`Since 7.1.0`.
+
       Although VNC doesn't support OpenGL natively, it can be paired with
       graphics type ``egl-headless`` (see below) which will instruct QEMU to
       open and use drm nodes for OpenGL rendering.

docs/schemas/domaincommon.rng

@@ -3663,6 +3663,11 @@
                   </choice>
                 </attribute>
               </optional>
+              <optional>
+                <attribute name="powerControl">
+                  <ref name="virYesNo"/>
+                </attribute>
+              </optional>
             </group>
             <group>
               <optional>