init/functions: do not guess 'eth0' as Qubes-managed interface

... if it doesn't exist.
The /qubes-mac qubesdb entry is present on Qubes 4.1, but not 4.0. It is
ok to depend on it here, but keep safer fallback if this code would need
to be backported.

init/functions

@@ -141,7 +141,7 @@ get_qubes_managed_iface() {
     qubes_iface="$(get_iface_from_mac "$mac")"
     if [ "x$qubes_iface" != "x" ]; then
         echo "$qubes_iface"
-    else
+    elif [ -e /sys/class/net/eth0 ]; then
         echo eth0
     fi
 }