app/testpmd: workaround deadlock in hot-unplug callback

app/testpmd: workaround deadlock in hot-unplug callback

Because the user's callback is invoked in eal interrupt callback, the
interrupt callback need to be finished before it can be unregistered
when detaching device. So finish callback soon and use a deferred
removal to detach device is need.

It is a workaround, once the device detaching be moved into the eal in
the future, the deferred removal could be deleted. This patch aim to
add this workaround and refine the function name and the description to
be more explicit and comment the limitation.

Fixes: 2049c5113fe8 ("app/testpmd: use hotplug failure handler")

Signed-off-by: Jeff Guo <jia.guo@intel.com>