Rewrite of the virtual timer code to support both modes at the same timer.
The timers now have a new member 'realtime' and they are driven by the
selected engine. The VGA update timer and the status LED timer now always use
the realtime mode, but the PIT and CMOS RTC depend on the clock options.


git-svn-id: svn://svn.code.sf.net/p/bochs/code/trunk@12514 6c8e4198-a580-4521-a02f-c9ae86df1db9