libxl: suspend: Return correct error from callbacks

libxl: suspend: Return correct error from callbacks

If a suspend callback fails, it has a libxl error code in its hand.
However we must return to libxc the values that libxc expects.  So we
stash the libxl error code in dss->rc and fish it out again after
libxc returns from the suspend call.

While we're here, abolish the now-redundant `ok' variable in
remus_devices_postsuspend_cb.

The overall functional change is that libxl_domain_save now completes
with the correct error code as determined when the underlying failure
happened.  (Usually this is, still, ERROR_FAIL.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
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v2: Add cleanup in remus_devices_postsuspend_cb.