KVM: PIT: provide an option to disable interrupt reinjection

Certain clocks (such as TSC) in older 2.6 guests overaccount for lost
ticks, causing severe time drift. Interrupt reinjection magnifies the
problem.

Provide an option to disable it.

[avi: allow room for expansion in case we want to disable reinjection
      of other timers]

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>