KVM: i8254: add kvm_pit_reset_reinject

pit_state.pending and pit_state.irq_ack are always reset at the same
time.  Create a function for them.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>