guest: Default to video type=virtio for machvirt

That's basically the only working config on KVM

tests/cli-test-xml/compare/virt-install-aarch64-graphics.xml

@@ -24,6 +24,9 @@
     <channel type="spicevmc">
       <target type="virtio" name="com.redhat.spice.0"/>
     </channel>
+    <video>
+      <model type="virtio"/>
+    </video>
     <rng model="virtio">
       <backend model="random">/dev/urandom</backend>
     </rng>

virtinst/guest.py

@@ -1224,6 +1224,9 @@ class Guest(XMLBuilder):
             self._add_spice_usbredir()
 
         video_model = self._os_object.default_videomodel(self)
+        if self.os.is_arm_machvirt():
+            video_model = "virtio"
+
         for video in self.get_devices("video"):
             if video.model == video.MODEL_DEFAULT:
                 video.model = video_model