single vnc server surface

This patch removes the server surface from VncState and adds a single
server surface to VncDisplay for all the possible clients connected.
Each client maintains a different dirty bitmap in VncState.
The guest surface is moved to VncDisplay as well because we don't need
to track guest updates in more than one place.

This patch has been updated to handle CopyRect correctly and efficiently.

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