nixos/libvirt: avoid dependency on two qemu packages

Currently libvirt requires two qemu derivations: qemu and qemu_kvm which is just a truncated version of qemu (defined as qemu.override { hostCpuOnly = true; }).

This patch exposes an option virtualisation.libvirtd.qemuPackage which allows to choose which package to use:

 * pkgs.qemu_kvm if all your guests have the same CPU as host, or
 * pkgs.qemu which allows to emulate alien architectures (for example ARMV7L on X86_64), or
 * a custom derivation

virtualisation.libvirtd.enableKVM option is vague and could be deprecate in favor of virtualisation.libvirtd.qemuPackage, anyway it does allow to enable/disable kvm.

pkgs/development/libraries/libvirt/default.nix

@@ -4,7 +4,7 @@
 , iproute, iptables, readline, lvm2, utillinux, systemd, libpciaccess, gettext
 , libtasn1, ebtables, libgcrypt, yajl, pmutils, libcap_ng, libapparmor
 , dnsmasq, libnl, libpcap, libxslt, xhtml1, numad, numactl, perlPackages
-, curl, libiconv, gmp, xen, zfs, parted, qemu
+, curl, libiconv, gmp, xen, zfs, parted
 }:
 
 with stdenv.lib;
@@ -41,8 +41,6 @@ stdenv.mkDerivation rec {
 
     # the path to qemu-kvm will be stored in VM's .xml and .save files
     # do not use "''${qemu_kvm}/bin/qemu-kvm" to avoid bound VMs to particular qemu derivations
-    substituteInPlace src/qemu/qemu_capabilities.c \
-      --replace '"/usr/libexec/qemu-kvm"' '"/run/libvirt/nix-emulators/${if stdenv.isAarch64 then "qemu-system-aarch64" else "qemu-kvm"}"'
     substituteInPlace src/lxc/lxc_conf.c \
       --replace 'lxc_path,' '"/run/libvirt/nix-emulators/libvirt_lxc",'
   '' + ''
@@ -91,7 +89,7 @@ stdenv.mkDerivation rec {
     substituteInPlace $out/lib/systemd/system/libvirtd.service --replace /bin/kill ${coreutils}/bin/kill
     rm $out/lib/systemd/system/{virtlockd,virtlogd}.*
     wrapProgram $out/sbin/libvirtd \
-      --prefix PATH : ${makeBinPath [ iptables iproute pmutils numad numactl qemu ]}
+      --prefix PATH : /run/libvirt/nix-emulators:${makeBinPath [ iptables iproute pmutils numad numactl ]}
   '';
 
   enableParallelBuilding = true;