BACKPORT: vnc: fix copyrect screen corruption

When sending a copyrect command to the vnc client, we must also update
the local server surface.  Otherwise the server's and the client's idea
of the screen content run out of sync and screen updates don't work
correctly.

[ backport: uses ds_get_data() instead of direct dereference ]
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Glauber Costa <glommer@redhat.com>