vnc: fix lossy rect refreshing

The for loop in send_lossy_rect was totally wrong, and we can't
call vnc_set_bits() because it does not really do what it should.
Use vnc_set_bit() directly instead.

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