All of these four functions (virStreamRecvAll, virStreamSendAll,
virStreamSparseRecvAll, virStreamSparseSendAll) take one or more
callback functions that handle various aspects of streams.
However, if any of them fails no error is reported therefore
caller does not know what went wrong.

At the same time, we silently presumed callbacks to set errno on
failure. With this change we should document it explicitly as the
error is not properly reported.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>