Introduce GIC feature

Some platforms, like aarch64, don't have APIC but GIC. So there's
no reason to have <apic/> feature turned on. However, we are
still missing <gic/> feature. This commit introduces the feature
to XML parser and formatter, adds documentation and updates RNG
schema.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

docs/formatdomain.html.in

@@ -1439,6 +1439,7 @@
       <hidden state='on'/>
     </kvm>
     <pvspinlock/>
+    <gic version='2'/>
 
   </features>
   ...</pre>
@@ -1545,6 +1546,15 @@
         the emulation of VMWare IO port, for vmmouse etc.
         <span class="since">Since 1.2.16</span>
       </dd>
+      <dt><code>gic</code></dt>
+      <dd>Enable for architectures using a General Interrupt
+          Controller instead of APIC in order to handle interrupts.
+          For example, the 'aarch64' architecture uses
+          <code>gic</code> instead of <code>apic</code>. The optional
+          attribute <code>version</code> specifies the GIC version;
+          however, it may not be supported by all hypervisors. <span
+          class="since">Since 1.2.16</span>
+      </dd>
     </dl>
 
     <h3><a name="elementsTime">Time keeping</a></h3>