libxl: suspend: switch_logdirty_done takes rc

libxl: suspend: switch_logdirty_done takes rc

switch_logdirty_done used to take the value to pass to
libxl__xc_domain_saverestore_async_callback_done (ie, the return value
from the callback).  (This was mistakenly described as "ok" in the
prototype, but in the definition it is "broke" and all the call sites
passed 0 for success or -1 for error.)

Instead, make it take a libxl error code (rc).  Convert this to the
suspend callback value at the end.

No functional change in this patch.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>