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>

docs/specs/libxl-migration-stream.pandoc

@@ -119,7 +119,9 @@ type         0x00000000: END
 
              0x00000003: EMULATOR_CONTEXT
 
-             0x00000004 - 0x7FFFFFFF: Reserved for future _mandatory_
+             0x00000004: CHECKPOINT_END
+
+             0x00000005 - 0x7FFFFFFF: Reserved for future _mandatory_
              records.
 
              0x80000000 - 0xFFFFFFFF: Reserved for future _optional_
@@ -203,3 +205,13 @@ index            Index of this emulator for the domain, if multiple
 
 emulator_ctx     Emulator context blob.
 --------------------------------------------------------------------
+
+CHECKPOINT\_END
+---------------
+
+A checkpoint end record marks the end of a checkpoint in the image.
+
+     0     1     2     3     4     5     6     7 octet
+    +-------------------------------------------------+
+
+The end record contains no fields; its body_length is 0.