Every time a client aborts a stream via the virStreamAbort API,
the daemon always logs an error like:

  error : daemonStreamHandleAbort:617 : stream aborted at client request

and that same error is returned to the client. Meaning virStreamAbort
always returns -1, which seems strange.

This reworks the error handling to only raise an error on virStreamAbort
if the actual server side abort call raises an error. This is similar
to how virStreamFinish works.

If the abort code path is triggered by an unexpected message type
then we continue to raise an unconditional error. Also drop a redundant
VIR_WARN call there, since virReportError will raise a VIR_ERROR anyways