[microvm] separate enableHostBridge from deployHosts

machines/modules/malobeo/microvm_host.nix

@@ -4,7 +4,6 @@ with lib;
 
 let
   cfg = config.services.malobeo.microvm;
-  hostsEmpty = length cfg.deployHosts == 0;
 in
 {
   options = {
@@ -56,6 +55,8 @@ in
         } ];
       };
 
+    } // mkIf (!hostEmpty) {
+
       # connect the vms to the bridge
       networks."11-microvm" = {
         matchConfig.Name = "vm-*";