vmm.4: Add ppt device detach example

Showcase how to detach ppt from a PCI device and attach a host driver,
and vice-versa.

MFC after: 2 days
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D46811

(cherry picked from commit 9ad2891558729b1c1ad4ba02377b157e404a3da2)