remus: resume immediately if libxl__xc_domain_save_done() completes

remus: resume immediately if libxl__xc_domain_save_done() completes

For example: if the secondary host is down, and we fail to send the data to
the secondary host. xc_domain_save() returns 0. So in the function
libxl__xc_domain_save_done(), rc is 0 (the helper program exits normally),
and retval is 0 (it is xc_domain_save()'s return value). In such case, we
just need to complete the stream.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- updated wording of comment as discussed ]