(#4) backup qcow image configuration

if blockdev-backup is used, the backup process must create the
target image using blockdev-add/create. In order to correctly
backup the image, we also require the original image config.

For now, store the image config in the backup folder alongside
the disk data.

libqmpbackup/image.py

@@ -18,6 +18,18 @@
 log = logging.getLogger(__name__)
 
 
+def get_info(filename):
+    """Query original qemu image information, can be used to re-create
+    the image during rebase operation with the same options as the
+    original one."""
+    try:
+        return subprocess.check_output(
+            ["qemu-img", "info", f"{filename}", "--output", "json", "--force-share"]
+        )
+    except subprocess.CalledProcessError as errmsg:
+        raise RuntimeError from errmsg
+
+
 def rebase(directory, dry_run, until):
     """Rebase and commit all images in a directory"""
     if not os.path.exists(directory):

qmpbackup

@@ -22,6 +22,7 @@ from qemu.qmp import protocol, QMPClient
 from libqmpbackup.qmpcommon import QmpCommon
 from libqmpbackup.lib import QmpBackup
 from libqmpbackup import vm
+from libqmpbackup import image
 from libqmpbackup import version
 
 SIGNAL_CATCHED = False
@@ -330,6 +331,17 @@ async def main():
                 common.thaw(qga)
             sys.exit(1)
 
+        for dev in blockdev:
+            infofile = f"{backupdir}/{dev.node}.config"
+            try:
+                info = image.get_info(dev.filename)
+            except RuntimeError as errmsg:
+                log.warning("Unable to get qemu image info: [%s]", errmsg)
+                continue
+            with open(infofile, "wb+") as info_file:
+                info_file.write(info)
+                log.info("Saved image info: [%s]", infofile)
+
         if argv.level == "copy":
             blockdev = vm.get_block_devices(
                 await qemu_client.do_query_block(), excluded_disks, included_disks