PCI hot-plug: use dedicated taskqueue for device attach / detach

Attaching and detaching devices can be heavy-weight and detaching can
sleep waiting for events. For that reason using the system-wide
single-threaded taskqueue_thread is not really appropriate.
There is even a possibility for a deadlock if taskqueue_thread is used
for detaching.

In fact, there is an easy to reproduce deadlock involving nvme, pass
and a sudden removal of an NVMe device.
A pass peripheral would not release a reference on an nvme sim until
pass_shutdown_kqueue() is executed via taskqueue_thread. But the
taskqueue's thread is blocked in nvme_detach() -> ... -> cam_sim_free()
because of the outstanding reference.

MFC after: 10 days
Sponsored by: CyberSecure
Reviewed by: mav, imp
Differential Revision: https://reviews.freebsd.org/D30144