tools/libxl: Extra management APIs for the save helper

tools/libxl: Extra management APIs for the save helper

With migration v2, there are several moving parts needing to be
juggled at once.  This requires the error handling logic to be able to
query the state of each moving part, possibly before they have been
started, and be able to cancel them.

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>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
v4: Don't force _init() to be mandatory
v3: Adjust helper_{stop,failed,done} to use libxl__save_helper_inuse()
v2: Add an _init() function which allows _inuse() to be safe to call even
    before the save helper has started.