svga: add guest statistic gathering interface

svga: add guest statistic gathering interface

With this patch, guest statistic gathering interface is added to
svga winsys interface that can be used to gather svga driver
statistic. The winsys module can then share the statistic info with
the VMX host via the mksstats interface.

The statistic enums used in the svga driver are defined in
svga_stats_count and svga_stats_time in svga_winsys.h

Reviewed-by: Brian Paul <brianp@vmware.com>