tools/libx{c,l}: Introduce restore_callbacks.checkpoint()

tools/libx{c,l}: Introduce restore_callbacks.checkpoint()

And call it when a checkpoint record is found in the libxc stream.

Some parts of this patch have been based on patches from the COLO
series.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
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>
---
v3: Named constants for the API
v2: Borrow sufficient fragments from several COLO patches to get
    BROKEN_CHANNEL and checkpoint failover to function.