tools/libxl: Write checkpoint records into the stream

tools/libxl: Write checkpoint records into the stream

when signalled to do so by libxl__remus_domain_checkpoint_callback()

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: Corrections to comments

This patch has changed substantially in v2 as a result of changes earlier in
the series.  No behavioural difference from v1.