libxc/migration: Specification update for CHECKPOINT records

libxc/migration: Specification update for CHECKPOINT records

Checkpointed streams need to signal the end of a consistent view of VM state,
and the start of the libxl data.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>