tools: handle xl migrate --debug in legacy stream

Doing a 'xl migrate --debug domU host' on xen-4.5 adds a
XC_SAVE_ID_ENABLE_VERIFY_MODE marker, which is not handled.
Since using --debug is valid usage, handle it by logging the fact
instead of aborting the migration.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit 281962851f)

tools/python/scripts/convert-legacy-stream

@@ -389,8 +389,7 @@ def read_chunks(vm):
             write_page_data(pfns, pages)
 
         elif marker == legacy.CHUNK_enable_verify_mode:
-            # For debugging purposes only.  Will not be seen in real migration
-            raise RuntimeError("Unable to convert a debug stream")
+            info("This is a debug stream")
 
         elif marker == legacy.CHUNK_vcpu_info:
             max_id, = unpack_exact("i")