manually impl Deserialize for PciPath for validation purposes

[gjcolombo]

Fixes #800.

[gjcolombo]

Just for fun, I also wrote and ran an exhaustive version of the deserialization test (for each bus/device/function in 0..256, call PciPath::new, then format a JSON version of that path and see if deserialization produces a result that matches what new returned). This takes about a minute to run on my Zen 2 dev machine, which is slow enough that I'm reluctant to check it in, especially given that this code doesn't change very often. But wanted to note that this seems to work for all otherwise-well-formed BDF JSON blobs.