Revert r5636 to fix icount on hosts w/o high-res clocks

Revert r5636 to fix icount on hosts w/o high-res clocks

Changeset r5636 changed the timers to run in the alarm callback.  The
alarm callback can only be called as frequently as the host alarm timer
fires.  For older Linux hosts and possibly non-Linux hosts, this can be
as high as a 1ms granularity.

icount calculates the select timeout based on the next deadline and
select is usually capable of sleeping for a short period of time than
alarm timer granularity.  This means that changing the timer callbacks
to be based on alarm firing caused timers to fire much later than they
ought to when using icount.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5796 c046a42c-6fe2-441c-8c8c-71466251a162