KVM: ioapic: reinject pending interrupts on KVM_SET_IRQCHIP

After the previous patches, an interrupt whose bit is set in the IRR
register will never be in the LAPIC's IRR and has never been injected
on the migration source.  So inject it on the destination.

This fixes migration of Windows guests without HPET (they use the RTC
to trigger the scheduler tick, and lose it after migration).

Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>