migration: Fix parsing of s390 stream

The parsing for the S390StorageAttributes section is currently leaving
an unconsumed token that is later interpreted by the generic code as
QEMU_VM_EOF, cutting the parsing short.

The migration will issue a STATTR_FLAG_DONE between iterations, which
the script consumes correctly, but there's a final STATTR_FLAG_EOS at
.save_complete that the script is ignoring. Since the EOS flag is a
u64 0x1ULL and the stream is big endian, on little endian hosts a byte
read from it will be 0x0, the same as QEMU_VM_EOF.

Fixes: 81c2c9dd5d ("tests/qtest/migration-test: Fix analyze-migration.py for s390x")
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20250109185249.23952-4-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit 69d1f784569fdb950f2923c3b6d00d7c1b71acc1)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

scripts/analyze-migration.py

@@ -65,6 +65,9 @@ class MigrationFile(object):
     def tell(self):
         return self.file.tell()
 
+    def seek(self, a, b):
+        return self.file.seek(a, b)
+
     # The VMSD description is at the end of the file, after EOF. Look for
     # the last NULL byte, then for the beginning brace of JSON.
     def read_migration_debug_json(self):
@@ -272,11 +275,24 @@ class S390StorageAttributes(object):
         self.section_key = section_key
 
     def read(self):
+        pos = 0
         while True:
             addr_flags = self.file.read64()
             flags = addr_flags & 0xfff
-            if (flags & (self.STATTR_FLAG_DONE | self.STATTR_FLAG_EOS)):
+
+            if flags & self.STATTR_FLAG_DONE:
+                pos = self.file.tell()
+                continue
+            elif flags & self.STATTR_FLAG_EOS:
                 return
+            else:
+                # No EOS came after DONE, that's OK, but rewind the
+                # stream because this is not our data.
+                if pos:
+                    self.file.seek(pos, os.SEEK_SET)
+                    return
+                raise Exception("Unknown flags %x", flags)
+
             if (flags & self.STATTR_FLAG_ERROR):
                 raise Exception("Error in migration stream")
             count = self.file.read64()