KVM: PIT: unregister kvm irq notifier if fail to create pit

If fail to create pit, we should unregister kvm irq notifier
which register in kvm_create_pit().

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>