daemonStreamHandleWriteData: Preserve error when aborting stream

daemonStreamHandleWriteData: Preserve error when aborting stream

The daemonStreamHandleWriteData() function is called whenever
server side of stream is able to receive some data. Nevertheless,
it calls virStreamSend() (to pass data down to virFDStream) and
depending on its return value it may abort the stream. However,
the functions it called when doing so are public APIs and as such
reset any error set previously. Therefore, if there was any error
in writing data to stream (i.e. repored in virStreamSend) it is
reset before virNetServerProgramSendReplyError() can get to it.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>