qemu, runInLinuxVM: fix KVM availability check

KVM should only be considered abailable if /dev/kvm exists and
is read-writable by the user that is trying to launch it.

The previous check for existance only had the consequence that
on some Linux distributions running VMs with Nix's QEMU only worked
if KVM was NOT installed.

fixes #124371

pkgs/applications/virtualization/qemu/default.nix

@@ -231,7 +231,7 @@ stdenv.mkDerivation rec {
     if [ -x $out/bin/qemu-system-${stdenv.hostPlatform.qemuArch} ]; then
       makeWrapper $out/bin/qemu-system-${stdenv.hostPlatform.qemuArch} \
                   $out/bin/qemu-kvm \
-                  --add-flags "\$([ -e /dev/kvm ] && echo -enable-kvm)"
+                  --add-flags "\$([ -r /dev/kvm -a -w /dev/kvm ] && echo -enable-kvm)"
     fi
   '';