Fix VMware VGA depth computation

VMware VGA requires that the depth presented to the guest is the same as the
DisplaySurface that it renders to.  This is because it performs a very simple
memcpy() to blit from one surface to another.

We currently hardcode a 24-bit depth.  The surface allocator for SDL may, and
usually will, allocate a surface with a different depth causing screen
corruption.

This changes the code to allocate the DisplaySurface before initializing the
device which allows the depth of the DisplaySurface to be used instead of
hardcoding something.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
