vmTools: omit '-drive ...' entirely instead of using /dev/null

Fixes #33378.

pkgs/build-support/vm/default.nix

@@ -208,7 +208,7 @@ rec {
       -device virtio-rng-pci \
       -virtfs local,path=${storeDir},security_model=none,mount_tag=store \
       -virtfs local,path=$TMPDIR/xchg,security_model=none,mount_tag=xchg \
-      -drive file=$diskImage,if=virtio,cache=unsafe,werror=report \
+      ''${diskImage:+-drive file=$diskImage,if=virtio,cache=unsafe,werror=report} \
       -kernel ${kernel}/${img} \
       -initrd ${initrd}/initrd \
       -append "console=ttyS0 panic=1 command=${stage2Init} out=$out mountDisk=$mountDisk loglevel=4" \
@@ -223,8 +223,6 @@ rec {
     mkdir xchg
     mv saved-env xchg/
 
-    diskImage=''${diskImage:-/dev/null}
-
     eval "$preVM"
 
     if [ "$enableParallelBuilding" = 1 ]; then
@@ -240,7 +238,7 @@ rec {
     # the -K option to preserve the temporary build directory).
     cat > ./run-vm <<EOF
     #! ${bash}/bin/sh
-    diskImage=$diskImage
+    ''${diskImage:+diskImage=$diskImage}
     TMPDIR=$TMPDIR
     cd $TMPDIR
     ${qemuCommand}