tools/libxl: rename remus checkpoint callbacks

tools/libxl: rename remus checkpoint callbacks

There are 2 remus checkpoint callbacks(save/restore), currently, they
both called libxl__remus_domain_checkpoint_callback in different
files, so it is ok. But in the following patch, we will move all of the
remus callback code into a separate file, the name should be different.
So rename them to:
  libxl__remus_domain_{save/restore}_checkpoint_callback

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>