baloonRatio: reexpose it from the vm definition

This is to allow downstream to override it as it necessary for their use
cases. Currently it is fixed and immutable from outside.

Signed-off-by: Brian McGillion <bmg.avoin@gmail.com>

Author: brianmcgillion

modules/microvm/vm-config.nix

@@ -97,6 +97,16 @@ let
         description = "App VM vCPU count.";
       };
 
+      balloonRatio = mkOption {
+        type = types.nullOr types.int;
+        default = null;
+        description = ''
+          Memory balloon ratio. The VM is allocated ramMb * (balloonRatio + 1)
+          bytes of memory, with ballooning enabled when balloonRatio > 0.
+          If null, uses the default from the VM definition (typically 2).
+        '';
+      };
+
       extraModules = mkOption {
         type = types.listOf types.unspecified;
         default = [ ];

modules/profiles/laptop-x86.nix

@@ -218,7 +218,8 @@ in
             effectiveDef =
               vmDef
               // lib.optionalAttrs ((vmCfg.ramMb or null) != null) { inherit (vmCfg) ramMb; }
-              // lib.optionalAttrs ((vmCfg.cores or null) != null) { inherit (vmCfg) cores; };
+              // lib.optionalAttrs ((vmCfg.cores or null) != null) { inherit (vmCfg) cores; }
+              // lib.optionalAttrs ((vmCfg.balloonRatio or null) != null) { inherit (vmCfg) balloonRatio; };
           in
           lib.nixosSystem {
             modules = [