guest: Make default graphics arch check more specific

We enable default graphics for all of ppc64, but really the only
thing we have ever tested is pseries, so make that clear.

Signed-off-by: Cole Robinson <crobinso@redhat.com>

virtinst/guest.py

@@ -927,7 +927,8 @@ class Guest(XMLBuilder):
             return
         if self.os.is_container() and not self.conn.is_vz():
             return
-        if self.os.arch not in ["x86_64", "i686", "ppc64", "ppc64le"]:
+        if (not self.os.is_x86() and
+            not self.os.is_pseries()):
             return
         self.add_device(DeviceGraphics(self.conn))