bhyve: Support other schemes for naming pass-through devices.

Permit naming pass through devices using the syntax accepted by
pciconf (pci[<domain>:]<bus>:<slot>:<func>) as well as by device name
(e.g. "ppt0").

While here, fix an error in the manpage that had the bus and slot
arguments for the original /-delimited scheme swapped.

Reviewed by: imp, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36147

(cherry picked from commit baf753cc196e2308828c7fb9b0385d5fe17ecdc2)