tools/libxl: Handle checkpoint records in a libxl migration v2 stream

tools/libxl: Handle checkpoint records in a libxl migration v2 stream

This is the final bit of untangling for Remus.

When libxc issues a checkpoint callback, start reading and buffering
all libxl records from the stream.  Once a CHECKPOINT_END record is
encountered, start processing all records.

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>
---
v4: Adjustments in line with review for earlier patches in the series
v3: Simplify, use named constants for API