conf: Add 'x' and 'y' resolution into video XML definition

This commit adds resolution element with parameters 'x' and 'y' into video
XML domain group definition. Both, properties were added into an element
called 'resolution' and it was added inside 'model' element. They are set
as optional. This element does not follow QEMU properties 'xres' and
'yres' format. Both HTML documentation and schema were changed too. This
commit includes a simple test case to cover resolution for QEMU video
models. The new XML format for resolution looks like:

    <model ...>
      <resolution x='800' y='600'/>
    </model>

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Julio Faracco <jcfaracco@gmail.com>

docs/formatdomain.html.in

@@ -7077,7 +7077,10 @@ qemu-kvm -net nic,model=? /dev/null
           <code>vgamem</code> (<span class="since">since 1.2.11</span>) to set
           the size of VGA framebuffer for fallback mode of QXL device.
           Attribute <code>vram64</code> (<span class="since">since 1.3.3</span>)
-          extends secondary bar and makes it addressable as 64bit memory.
+          extends secondary bar and makes it addressable as 64bit memory. For
+          resolution settings, there are <code>x</code> and <code>y</code>
+          (<span class="since">since 5.9.0</span>) optional attributes to set
+          minimum resolution for model.
         </p>
       </dd>
 

docs/schemas/domaincommon.rng

@@ -3656,6 +3656,16 @@
               </optional>
             </element>
           </optional>
+          <optional>
+            <element name="resolution">
+              <attribute name="x">
+                <ref name="unsignedInt"/>
+              </attribute>
+              <attribute name="y">
+                <ref name="unsignedInt"/>
+              </attribute>
+            </element>
+          </optional>
         </element>
       </optional>
       <optional>