guest: Don't add qemu-ga on ARM where virtio slots are limited

tests/cli-test-xml/compare/arm-vexpress-f19.xml

@@ -33,9 +33,5 @@
       <model type="virtio"/>
     </interface>
     <console type="pty"/>
-    <channel type="unix">
-      <source mode="bind"/>
-      <target type="virtio" name="org.qemu.guest_agent.0"/>
-    </channel>
   </devices>
 </domain>

virtinst/guest.py

@@ -567,7 +567,9 @@ class Guest(XMLBuilder):
         if self.get_devices("channel"):
             return
 
+        # Skip qemu-ga on ARM where virtio slots are currently limited
         if (self.conn.is_qemu() and
+            not self.os.is_arm() and
             self._lookup_osdict_key("qemu_ga", False) and
             self.conn.check_support(self.conn.SUPPORT_CONN_AUTOSOCKET)):
             dev = virtinst.VirtualChannelDevice(self.conn)