nixos/qemu-flags: Take a 'pkgs' parameter

I'm gonna use it in the next commit.

nixos/lib/build-vms.nix

···
 let pkgs = import ../.. { inherit system config; }; in
 
 with pkgs.lib;
-with import ../lib/qemu-flags.nix;
+with import ../lib/qemu-flags.nix { inherit pkgs; };
 
 rec {
 

nixos/lib/qemu-flags.nix

···
 # QEMU flags shared between various Nix expressions.
+{ pkgs }:
 
 {