conf: Add gl property to graphics of type sdl in domain config

Support OpenGL accelerated rendering when using SDL graphics in the
domain config. Add associated test and documentation.

Signed-off-by: Maciej Wolny <maciej.wolny@codethink.co.uk>
Reviewed-by: John Ferlan <jferlan@redhat.com>

docs/schemas/domaincommon.rng

@@ -3010,6 +3010,14 @@
               <ref name="virYesNo"/>
             </attribute>
           </optional>
+          <optional>
+            <element name="gl">
+              <attribute name="enable">
+                <ref name="virYesNo"/>
+              </attribute>
+              <empty/>
+            </element>
+          </optional>
         </group>
         <group>
           <attribute name="type">