calling stipending() is only needed in the PV case; in PVHVM
hypervisor_callback() will be called again as soon as we enable interrupts.

Use the PVHVM XENINTRSTUB for PV too; asjust Xdoreti as needed.
merge hypervisor_callback with hypervisor_pvhvm_callback. The reetrancy should
not be an issue, as we're already re-enabling events in
do_hupervisor_callback (that it, outside of the critical section).
We now can call Xdoreti in both cases.

(bouyer)