Use readonly=on for EFI flash device

Signed-off-by: Marc Zyngier <maz@kernel.org>
diff --git a/runefivm.sh b/runefivm.sh
index 6525b37..7a8d4ca 100755
--- a/runefivm.sh
+++ b/runefivm.sh
@@ -104,7 +104,7 @@
 
 args="  -m $ram -smp $cpus -cpu $cputype,aarch64=$sf -machine virt,accel=$accel,gic-version=$gic \
 	-nographic \
-	-drive if=pflash,format=raw,readonly,file=$efi $efivarpflash \
+	-drive if=pflash,format=raw,readonly=on,file=$efi $efivarpflash \
 	$netdev \
 	$dskdev \
 	$cddev \