Add -s "help" and -l "help" to print a list of supported PCI and LPC devices.

For tools that uses bhyve such like libvirt, it is important to be able to
probe what features are supported by the given bhyve binary.

To give more context, libvirt probes bhyve's capabilities in a not very
effective way:

• Running 'bhyve -h' and parsing output.
• To detect devices, it runs 'bhyve -s 0,dev' for every each device and parses error output to identify if the device is supported or not.

PR: 2101111
Submitted by: novel
MFC after: 2 weeks
Relnotes: yes
Sponsored by: iXsystems Inc.