Explicitly enable emulated GPU

Previously it was enabled only because of bug in libvirt. Now it is
fixed (9e0e0224438b79073bf404627fed8e2889669374), so config needs to be
updated.

vm-config/xen-vm-template-hvm.xml

@@ -29,6 +29,7 @@
 {netdev}
 {pcidevs}
     <input type='tablet' bus='usb'/>
+    <video type='vga'/>
   </devices>
 </domain>