nixos/qemu-vm: Update system.requiredKernelConfig

Verify that all kernel modules which are required for mounting
/nix/store in the VM are present.

nixos/modules/virtualisation/qemu-vm.nix

@@ -744,6 +744,8 @@ in
         (isEnabled "VIRTIO_PCI")
         (isEnabled "VIRTIO_NET")
         (isEnabled "EXT4_FS")
+        (isEnabled "NET_9P_VIRTIO")
+        (isEnabled "9P_FS")
         (isYes "BLK_DEV")
         (isYes "PCI")
         (isYes "NETDEVICES")
@@ -753,6 +755,8 @@ in
       ] ++ optionals (!cfg.graphics) [
         (isYes "SERIAL_8250_CONSOLE")
         (isYes "SERIAL_8250")
+      ] ++ optionals (cfg.writableStore) [
+        (isEnabled "OVERLAY_FS")
       ];
 
   };