dom0-updates: prefer yum-deprecated over dnf

Some of the reasons:
 - dnf doesn't support --downloaddir option
 - dnf doesn't support `copy_local` repo option (used in automated tests
   only)
 - dnf is horribly slow, especially without cache fetched
 (https://bugzilla.redhat.com/show_bug.cgi?id=1227014)

This is all needed (instead of simply using `yum` command), because
Fedora >= 22 have an command redirection `yum`->`dnf`.

QubesOS/qubes-issues#1282

misc/qubes-download-dom0-updates.sh

@@ -104,9 +104,12 @@ if [ "$DOIT" != "1" -a "$PKGS_FROM_CMDLINE" != "1" ]; then
 fi
 
 YUM_COMMAND="fakeroot yum $YUM_ACTION -y --downloadonly --downloaddir=$DOM0_UPDATES_DIR/packages"
+# prefer yum-deprecated over dnf, because of still missing features in dnf (at least --downloaddir)
+if type dnf >/dev/null 2>&1 && type yum-deprecated >/dev/null 2>&1; then
+    YUM_COMMAND="fakeroot yum-deprecated $YUM_ACTION -y --downloadonly --downloaddir=$DOM0_UPDATES_DIR/packages"
 # check for --downloadonly option - if not supported (Debian), fallback to
 # yumdownloader
-if ! yum --help | grep -q downloadonly; then
+elif ! yum --help | grep -q downloadonly; then
     if [ "$YUM_ACTION" != "install" -a "$YUM_ACTION" != "upgrade" ]; then
         echo "ERROR: yum version installed in VM `hostname` does not suppport --downloadonly option" >&2
         echo "ERROR: only 'install' and 'upgrade' actions supported ($YUM_ACTION not)" >&2