support setting network device names

this means that net devices in devices/foo/default.nix can be
specified by their sysfs paths (instead of by "eth0" and "eth1" that
may change from one kernel version to the next) and given mnenomic
names that are helpful for the hardware. Like "wan" and "lan[1..4]"

devices/qemu/default.nix

@@ -77,8 +77,14 @@
         networkInterfaces =
           let inherit (config.system.service.network) link;
           in {
-            wan = link.build { ifname = "eth0"; };
-            lan = link.build { ifname = "eth1"; };
+            wan = link.build {
+              devpath = "/devices/pci0000:00/0000:00:13.0/virtio0";
+              ifname = "wan";
+            };
+            lan = link.build {
+              devpath = "/devices/pci0000:00/0000:00:14.0/virtio1";
+              ifname = "lan";
+            };
 
             wlan_24 = link.build {
               ifname = "wlan0";