Make Kernel Samepage Merging optional

As MicroVMs work fine without KSM enabled, it should probably be
optional. While KSM can improve a lot of workloads, it can harm
others, and so this configuration should not conflict with a value
explicitly set by the user.

nixos-modules/host.nix

@@ -466,6 +466,6 @@ in
     '';
 
     # Enable Kernel Same-Page Merging
-    hardware.ksm.enable = true;
+    hardware.ksm.enable = lib.mkDefault true;
   };
 }