Fix SIGFPE for vnc display of width/height = 1

During boot, the screen gets resized to height 1 and a mouse click at this
point will cause a division by zero when calculating the absolute pointer
position from the pixel (x, y). Return a click in the middle of the screen
instead in this case.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
