vnc fixes and improvements (Stefano Stabellini)

vnc fixes and improvements (Stefano Stabellini)

this patch fixes a bug and improves the generic pixel conversion
function in vnc.c.
The bug is that when a new vnc client connects we need to reset the flag
has_WMVi but currently we don't.
The generic pixel conversion function is vnc_convert_pixel and currently
is not very efficient since uses the division and multiplication
operators.
To make it more efficient I changed to use bit shift operators instead.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6441 c046a42c-6fe2-441c-8c8c-71466251a162