Introduce virStreamSendHole

Introduce virStreamSendHole

This API is used to tell the other side of the stream to skip
some bytes in the stream. This can be used to create a sparse
file on the receiving side of a stream.

It takes @length argument, which says how big the hole is. This
skipping is done from the current point of stream. Since our
streams are not rewindable like regular files, we don't need
@whence argument like seek(2) has.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>