qemu: emit warnings when KVM acceleration is not usable
qemu: emit warnings when KVM acceleration is not usable

This fixes the qemu-kvm wrapper we add for convenience
silently not using KVM, when the system would support it
by at least leaving an indication in the log that the build ran
slower because it ran without KVM.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs