nvme_sim: Only report PCI related stats when we can

For AHCI attached devices, we report the location and identification
information of the AHCI controller that we're attached to. We also
don't reprot link speed in that case, since we can't get to the PCIe
config space registers to find that out.

Sponsored by: Netflix
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D33287