vfio: handle hotplug request notifier

vfio: handle hotplug request notifier

When device is be hot-unplugged, the vfio kernel module will sent req
notifier to request user space to release the allocated resources at
first. After that, vfio kernel module will detect the device disappear,
and then delete the device in kernel.

This patch aim to add req notifier processing to enable hotplug for vfio.
By enable the req notifier monitoring and register the notifier callback,
when device be hot-unplugged, the hot-unplug handler will be called to
process hotplug for vfio.

Signed-off-by: Jeff Guo <jia.guo@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>