Use only one build of qemu in VM tests

Previously we were using two or three (qemu_kvm, qemu_test, and
qemu_test with a different dbus when minimal.nix is included).

(cherry picked from commit 8bfa4ce82ea7d23a1d4c6073bcc044e6bf9c4dbe)

nixos/modules/virtualisation/qemu-vm.nix

@@ -13,6 +13,8 @@ with lib;
 
 let
 
+  qemu = config.system.build.qemu or pkgs.qemu_test;
+
   vmName =
     if config.networking.hostName == ""
     then "noname"
@@ -32,7 +34,7 @@ let
       NIX_DISK_IMAGE=$(readlink -f ''${NIX_DISK_IMAGE:-${config.virtualisation.diskImage}})
 
       if ! test -e "$NIX_DISK_IMAGE"; then
-          ${pkgs.qemu_kvm}/bin/qemu-img create -f qcow2 "$NIX_DISK_IMAGE" \
+          ${qemu}/bin/qemu-img create -f qcow2 "$NIX_DISK_IMAGE" \
             ${toString config.virtualisation.diskSize}M || exit 1
       fi
 
@@ -47,7 +49,7 @@ let
       ${if cfg.useBootLoader then ''
         # Create a writable copy/snapshot of the boot disk.
         # A writable boot disk can be booted from automatically.
-        ${pkgs.qemu_kvm}/bin/qemu-img create -f qcow2 -b ${bootDisk}/disk.img $TMPDIR/disk.img || exit 1
+        ${qemu}/bin/qemu-img create -f qcow2 -b ${bootDisk}/disk.img $TMPDIR/disk.img || exit 1
 
         ${if cfg.useEFIBoot then ''
           # VM needs a writable flash BIOS.
@@ -63,14 +65,14 @@ let
       extraDisks=""
       ${flip concatMapStrings cfg.emptyDiskImages (size: ''
         if ! test -e "empty$idx.qcow2"; then
-            ${pkgs.qemu_kvm}/bin/qemu-img create -f qcow2 "empty$idx.qcow2" "${toString size}M"
+            ${qemu}/bin/qemu-img create -f qcow2 "empty$idx.qcow2" "${toString size}M"
         fi
         extraDisks="$extraDisks -drive index=$idx,file=$(pwd)/empty$idx.qcow2,if=${cfg.qemu.diskInterface},werror=report"
         idx=$((idx + 1))
       '')}
 
       # Start QEMU.
-      exec ${pkgs.qemu_test}/bin/qemu-kvm \
+      exec ${qemu}/bin/qemu-kvm \
           -name ${vmName} \
           -m ${toString config.virtualisation.memorySize} \
           ${optionalString (pkgs.stdenv.system == "x86_64-linux") "-cpu kvm64"} \
@@ -121,7 +123,7 @@ let
               mkdir $out
               diskImage=$out/disk.img
               bootFlash=$out/bios.bin
-              ${pkgs.qemu_kvm}/bin/qemu-img create -f qcow2 $diskImage "40M"
+              ${qemu}/bin/qemu-img create -f qcow2 $diskImage "40M"
               ${if cfg.useEFIBoot then ''
                 cp ${pkgs.OVMF-CSM}/FV/OVMF.fd $bootFlash
                 chmod 0644 $bootFlash