docs/libxl: Introduce CHECKPOINT_END to support migration v2 remus streams

In a remus scenario, libxc will write a CHECKPOINT record, then hand
ownership of the fd to libxl.  Libxl then writes any records required
and finishes with a CHECKPOINT_END record, then hands ownership of the
fd back to libxc.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>

tools/python/xen/migration/libxl.py

@@ -36,12 +36,14 @@ REC_TYPE_end              = 0x00000000
 REC_TYPE_libxc_context    = 0x00000001
 REC_TYPE_xenstore_data    = 0x00000002
 REC_TYPE_emulator_context = 0x00000003
+REC_TYPE_checkpoint_end   = 0x00000004
 
 rec_type_to_str = {
     REC_TYPE_end              : "End",
     REC_TYPE_libxc_context    : "Libxc context",
     REC_TYPE_xenstore_data    : "Xenstore data",
     REC_TYPE_emulator_context : "Emulator context",
+    REC_TYPE_checkpoint_end   : "Checkpoint end",
 }
 
 # emulator_context
@@ -176,6 +178,13 @@ class VerifyLibxl(VerifyBase):
         self.info("  Index %d, type %s" % (emu_idx, emulator_id_to_str[emu_id]))
 
 
+    def verify_record_checkpoint_end(self, content):
+        """ Checkpoint end record """
+
+        if len(content) != 0:
+            raise RecordError("Checkpoint end record with non-zero length")
+
+
 record_verifiers = {
     REC_TYPE_end:
         VerifyLibxl.verify_record_end,
@@ -185,4 +194,6 @@ record_verifiers = {
         VerifyLibxl.verify_record_xenstore_data,
     REC_TYPE_emulator_context:
         VerifyLibxl.verify_record_emulator_context,
+    REC_TYPE_checkpoint_end:
+        VerifyLibxl.verify_record_checkpoint_end,
 }