assemblers/qemu: support for qcow2 compat version

Add a new option `qcow2_comapt` which can be used explicitly
select the compatibility level of the qcow2 file format. Qemu
version 1.1 introduced extensions to the format that became
the default with 1.7, which are not readable by qemu < 1.1.
Thus if the resulting qcow2 should be read by such older qemu
versions, the compatibility level needs to be set to 0.10.

assemblers/org.osbuild.qemu

@@ -59,6 +59,10 @@ SCHEMA = """
     "type": "string",
     "enum": ["raw", "raw.xz", "qcow2", "vdi", "vmdk", "vpc", "vhdx"]
   },
+  "qcow2_compat": {
+    "description": "The qcow2-compatibility-version to use",
+    "type": "string"
+  },
   "filename": {
     "description": "Image filename",
     "type": "string"
@@ -690,6 +694,11 @@ def main(tree, output_dir, options, loop_client):
             "vpc": ["-o", "subformat=fixed,force_size"],
             "vhdx": []
         }
+
+        compat = options.get("qcow2_compat")
+        if compat:
+            extra_args["qcow2"] += ["-o", f"compat={compat}"]
+
         subprocess.run([
             "qemu-img",
             "convert",