network: fix rules for network setup on new udev

New udev have `DRIVERS` matcher, instead of `ENV{ID_NET_DRIVER}`. Add
appropriate rule to the file. Without it, network was working
incidentally, because there is a fallback in qubes-misc-post.service,
but dynamic network change was broken.

This applies at least to Debian stretch.

Fixes QubesOS/qubes-issues#3192

network/udev-qubes-network.rules

@@ -1,2 +1,5 @@
 
+# old udev has ENV{ID_NET_DRIVER}
 SUBSYSTEMS=="xen", KERNEL=="eth*", ACTION=="add", ENV{ID_NET_DRIVER}=="vif", RUN+="/usr/lib/qubes/setup-ip"
+# new udev has DRIVERS
+SUBSYSTEMS=="xen", KERNEL=="eth*", ACTION=="add", DRIVERS=="vif", RUN+="/usr/lib/qubes/setup-ip"