pit: fix pit interrupt can't inject into vm after migration

pit: fix pit interrupt can't inject into vm after migration

kvm_pit is running in kmod. kvm_pit is going to inject
interrupt to vm before cpu_synchronize_all_post_init at
dest side. vcpu will lose the pit interrupt, but
ack_irq(in kmod) has been 0. ack_irq become 1 after
vcpu responds pit interrupt. pit interruptcan inject
to vm when ack_irq is 1.

By the way, kvm_pit_vm_state_change has save and load
state of pit, so pre_save and post_load is unnecessary.

Signed-off-by: ChenLiang <chenliang88@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>