virStreamInData: Allow callback to not rewind the stream

virStreamInData: Allow callback to not rewind the stream

So far, virStreamInData() is effectively a wrapper over
virFDStreamInData() which means it deals with files which can be
rewound (lseek()-ed) to whatever position we need. And in fact,
that's what virFDStreamInData() does - it makes sure that the FD
is left unchanged in terms of position in the file.  Skipping the
hole happens soon after - in daemonStreamHandleRead() when
virStreamSendHole() is called.

But this is about to change. Soon we will have another implementation
where we won't be dealing with FDs but virNetMessage queue and it will
be handy to pop message at the beginning of the queue. Implement and
document this new behavior.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>