create: customize: Fix inadvertent disk removal on install fail

If the first install attempt fails, then the second attempt succeeds,
we were still removing the disk images we created as though the
install never succeeded. We need to clear out the cached failed_guest
value via the customize dialog callback

https://bugzilla.redhat.com/show_bug.cgi?id=1342043

virtManager/create.py

@@ -2218,6 +2218,7 @@ class vmmCreate(vmmGObjectUI):
             if not self.is_visible():
                 return
             logging.debug("User finished customize dialog, starting install")
+            self._failed_guest = None
             guest.update_defaults()
             self._start_install(guest)