Prefer 'dnf' over 'yum' for template update

QubesOS/qubes-issues#1282

qubes-rpc/qubes.InstallUpdatesGUI

@@ -4,7 +4,9 @@
 # If you are creating package for other distribution, feel free to replace it
 # with distribution-specific script.
 
-if [ -e /etc/redhat-release ]; then
+if [ -e /etc/redhat-release -a -x /usr/bin/dnf ]; then
+    xterm -title update -e su -l -c 'dnf update --best; echo Done. Press Enter to exit.; read x'
+elif [ -e /etc/redhat-release -a -x /usr/bin/yum ]; then
     xterm -title update -e su -l -c 'yum update; echo Done. Press Enter to exit.; read x'
 elif [ -e /etc/debian_version ]; then
     xterm -title update -e su -l -c 'apt-get update && apt-get -V dist-upgrade; echo Done. Press Enter to exit.; read x'