run_qemu.sh: fix --rw option just broken by switch to -blockdev

Use a qcow2 overlay to fix the --rw regression just added by commit
dfa1d3f ("run_qemu.sh: replace -drive rootfs with -blockdev +
-device")

The QEMU -snapshot option is silently ignored by -blockdev (this is
documented in the qemu-system man page)

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Author: marc-hb

parser_generator.m4

@@ -38,7 +38,7 @@ exit 11  #)Created by argbash-init v2.9.0
 # ARG_OPTIONAL_BOOLEAN([gdb], , [Wait for gdb to connect for kernel debug (port 10000)], )
 # ARG_OPTIONAL_BOOLEAN([gdb-qemu], , [Start qemu with gdb], )
 # ARG_OPTIONAL_BOOLEAN([qmp], , [Invokes QEMU with -qmp which opens a QMP control socket at unix:/tmp/run_qemu_qmp. Using that socket requires the 'qmp-shell' script.\nWhen needed, run_qemu.sh can find qmp-shell either in the PATH, or in scripts/qmp/ in the qemu source when using --git-qemu.\nThe official qemu.qmp package is available on PyPI.], )
-# ARG_OPTIONAL_BOOLEAN([rw], , [Make guest image writeable (remove -snapshot)\n (Note that an image rebuild will lose any changes made via --rw)], )
+# ARG_OPTIONAL_BOOLEAN([rw], , [Persist run-time image changes for the next cold boot.\nA reboot does not lose changes; for persistency a reboot is a non-event.\nNote that an image rebuild will always reset any changes made via --rw]\n, )
 # ARG_OPTIONAL_BOOLEAN([curses], , [Default display is -nographic. switch to -curses instead with this option.\n Use Esc+1, Esc+2, etc. to switch between the different screens.\n 'q' in the monitor screen to quit], )
 # ARG_OPTIONAL_BOOLEAN([git-qemu], [g], [Use a qemu tree at '~/git/qemu/' for qemu binaries.\n This overrides any qemu=<foo> setting from the env.],  )
 # ARG_OPTIONAL_BOOLEAN([nfit-test], , [Setup an environment for unit tests\n  - include libnvdimm 'extra' modules\n  - add some memmap reserved memory\n Note: --rebuild=img or higher required when switching either to or away from nfit-test.\n This overrides any supplied 'preset' or topology options and forces preset=med], )

run_qemu.sh

@@ -1862,7 +1862,18 @@ prepare_qcmd()
 		;;
 	esac
 
-	qcmd+=("-blockdev" "driver=file,node-name=maindisk,filename=$_arg_rootfs")
+	if [[ $_arg_rw == 'on' ]]; then
+		qcmd+=("-blockdev" "driver=file,node-name=maindisk,filename=${_arg_rootfs}")
+	else
+		local _overlay=${_arg_rootfs}-overlay.qcow2
+		rm -f "${_overlay}"
+		qemu-img create -F raw -b "${_arg_rootfs}" -f qcow2 "${_overlay}"
+		# "Finding your way through the QEMU parameter jungle"
+		# -- Thomas Huth
+		qcmd+=("-blockdev" "file,node-name=mainoverlay,filename=${_overlay}")
+		qcmd+=("-blockdev" "qcow2,node-name=maindisk,file=mainoverlay")
+	fi
+
 	qcmd+=("-device"   "virtio-blk,bus=pcie.0,drive=maindisk")
 
 	if [ $_arg_direct_kernel = "on" ] && [ -n "$vmlinuz" ] && [ -n "$initrd" ]; then
@@ -1903,6 +1914,7 @@ prepare_qcmd()
 	fi
 
 	if [[ $_arg_rw == "off" ]]; then
+		# Note this is only for the (deprecated) -drive and ignored by -blockdev
 		qcmd+=("-snapshot")
 	fi