schema: List allowed GIC versions

This change allows to use "host" as a GIC version in the domain XML.

Since we'll need to update the virGICVersion enumeration to support
new GIC versions anyway, it makes sense to be a bit more strict in
the schema as well and reject values that are not in the enumeration.

docs/schemas/domaincommon.rng

@@ -4154,7 +4154,11 @@
             <element name="gic">
               <optional>
                 <attribute name="version">
-                  <ref name="positiveInteger"/>
+                  <choice>
+                    <value>host</value>
+                    <value>2</value>
+                    <value>3</value>
+                  </choice>
                 </attribute>
               </optional>
             </element>