vfio-pci: Enable device request notification support

vfio-pci: Enable device request notification support

Linux v4.0-rc1 vfio-pci introduced a new virtual interrupt to allow
the kernel to request a device from the user.  When signaled, QEMU
will by default attmempt to hot-unplug the device.  This is a one-
shot attempt with the expectation that the kernel will continue to
poll for the device if it is not returned.  Returning the device when
requested is the expected standard model of cooperative usage, but we
also add an option option to disable this feature.  Initially this
opt-out is set as an experimental option because we really should
honor kernel requests for the device.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>