KVM: Fix unbounded preemption latency

When preparing to enter the guest, if an interrupt comes in while
preemption is disabled but interrupts are still enabled, we miss a
preemption point.  Fix by explicitly checking whether we need to
reschedule.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Avi Kivity <avi@qumranet.com>