bus: add hot-unplug handler

bus: add hot-unplug handler

A hot-unplug failure and app crash can be caused, when a device is
hot-unplugged but the application still try to access the device
by reading or writing from the BARs, which is already invalid but
still not timely be unmap or released.

This patch introduces bus ops to handle hot-unplug failures. Each
bus can implement its own case-dependent logic to handle the failures.

Signed-off-by: Jeff Guo <jia.guo@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>