qubes-prepare-vm-kernel: ensure proper permissions on initramfs

By default dracut set 600 mode. While it isn't a problem for starting a
VM (which is done as root), it is a problem for creating standalone VM,
which has it's own copy of kernel directory.

(cherry picked from commit 2f1c8ea459)

kernel-modules/qubes-prepare-vm-kernel

@@ -56,6 +56,7 @@ function build_initramfs() {
         --conf /dev/null --confdir /var/empty \
         -d "xenblk xen-blkfront cdrom ext4 jbd2 crc16 dm_snapshot" \
         $output_file $kver
+    chmod 644 "$output_file"
 }
 
 if [ -z "$1" ]; then