dom0-updates: move PackageKit cache refresh to GUI handling

PackageKit is used only by GUI updaters, refresh its cache only then.
Since PackageKit daemon do not read dnf.conf, it doesn't know  the right
repository location, so try to access network, which results in
timeouts. But at the same time, it invalidate previous cache, which is
what we need.
Do not delay every qubes-dom0-update call by pkcon call.

dom0-updates/qubes-dom0-update

@@ -212,6 +212,8 @@ if [ -n "$PKGS" ]; then
 elif [ -f /var/lib/qubes/updates/repodata/repomd.xml ]; then
     # Above file exists only when at least one package was downloaded
     if [ "$GUI" == "1" ]; then
+        # refresh packagekit metadata, GUI utilities use it
+        pkcon refresh force
         $guiapp
     else
         dnf check-update

dom0-updates/qubes-receive-updates

@@ -116,14 +116,6 @@ def handle_dom0updates(updatevm):
     # Clean old cache
     subprocess.call(["sudo", "/usr/bin/yum", "-q", "clean", "all"],
         stdout=sys.stderr)
-    # This will fail because of "smart" detection of no-network,
-    # but it will invalidate the cache
-    try:
-        null = open('/dev/null', 'w')
-        subprocess.call(["/usr/bin/pkcon", "refresh"], stdout=null)
-        null.close()
-    except subprocess.CalledProcessError:
-        pass
     os.umask(old_umask)
     exit(0)