newvm: Add --network argument

Although the `newvm.sh` script had a variable to configure the value
specified for the `--network` argument to `virt-install`, it didn't
expose a way to set it.  We need this ability so we can e.g. create VMs
on non-default networks like `camera` or `mgmt`.

newvm.sh

@@ -58,6 +58,13 @@ while [ $# -gt 0 ]; do
         shift
         fedora="${1#*=}"
         ;;
+    --network)
+        shift
+        network="$1"
+        ;;
+    --network=*)
+        network="${1#*=}"
+        ;;
     --no-console|--noconsole)
         console=false
         ;;