Credit1: Tweak reset condition

Credit1: Tweak reset condition

VMs that don't use their full timeslice are guaranteed to flip back
and forth between "active" and "inactive".  If we set credit to 0
when setting "inactive", then when the VM comes back to "active"
again, it will effectively be behind most other vcpus in credit.
This causes the credit1 to effectively discriminate *against*
VMs which use less than their full timeslice.

Instead of setting credit to 0, divide it in half.  This gets rid of
some of the system credit while allowing non-cpu-bound VMs to keep
some priority advantage.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>