This ensures that we have processed all hotplug events before
considering enumeration to be complete. This is important due to USB
persist being turned off. At resume time, devices will disappear and
immediately re-appear. In this situatoin, enumerate could first see the
old state with a removed device resulting in it to not be discovered.
As a hotplug event is semingly emitted by the kernel immediately, we
can simply make sure to process this hotplug event before returning
from enumerate.
Closes: fprintd#119
05fd2c58cb