register device removed event

Now nova support detach a device dynamically, but it is not guaranteed to succeed.

What happens is that the hypervisor injects an ACPI request to unplug the device. The guest OS must co-operate by releasing the device, before the hypervisor will complete the action of physically removing it. So when require a guest OS that supports ACPI unplug of course, and if the guest is crashed or being malicious there is no guarantee the unplug will succeed. Libvirt will wait a short while for success, but nova must monitor for libvirt events to see if/when it finally completes. This delayed release has implications for when Nova can mark the PCI device as unused and available for other guests to assign.