spec: newer dracut does't support /dev/null as conf

See dracutdevs/dracut#1835

(cherry picked from commit 3c86f60)

kernel.spec.in

@@ -477,11 +477,12 @@ mv  %buildroot/lib/firmware-all %buildroot/lib/firmware/%kernelrelease
 
 # Prepare initramfs for Qubes VM
 mkdir -p %buildroot/%vm_install_dir
+touch /tmp/dummy_conf
 PATH="/sbin:$PATH" dracut --nomdadmconf --nolvmconf --no-hostonly \
     --kmoddir %buildroot/lib/modules/%kernelrelease \
     --modules "kernel-modules qubes-vm-simple busybox" \
     --omit "nss-softokn extra-modules qubes-pciback qubes-udev" \
-    --conf /dev/null --confdir /var/empty \
+    --conf /tmp/dummy_conf --confdir /var/empty \
     -d "xenblk xen-blkfront cdrom ext4 jbd2 crc16 dm_snapshot" \
     %buildroot/%vm_install_dir/initramfs %kernelrelease