vnc: return the number of rectangles

Some encodings like tight supports tiling (spliting in
multiple sub-rectangles). So we needed a way to tell
vnc_update_client() how much rectangles are in the buffer.

zlib, raw and hextile always send a full rectangle.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>