tools/libxl: Do not set stream->rc in stream_complete()

tools/libxl: Do not set stream->rc in stream_complete()

Only ever set stream->rc in check_all_finished().  The first version of
the migration v2 series had separate rc and joined_rc parameters, where
this logic worked.  However when combining the two, the teardown path
fails to trigger if stream_complete() records stream->rc itself.  A side
effect of this is that stream_done() needs to take an rc parameter.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>