microvm: fix default mounts

modules/microvm.nix

@@ -56,7 +56,6 @@ in
     mounts = mkOption {
       description = "Persistent filesystems to create, without leading /.";
       type = with types; listOf str;
-      default = [ "etc" "home" "var" ];
     };
 
     mountBase = mkOption {
@@ -67,6 +66,8 @@ in
   };
 
   config = {
+    c3d2.deployment.mounts = [ "etc" "home" "var" ];
+
     microvm = {
       hypervisor = lib.mkDefault "cloud-hypervisor";
       mem = lib.mkDefault 512;