Improve the PCIe configuration space interface

[zdivelbiss]

The primary focus here is improving the interface used by PlatformInfo to enumerate the PCIe configuration space. While the current enumeration via PciConfig::physical_address tends to be flexible enough for most implementations, a more involved implementation dealing with multi-socket or generally separated PCI host controllers (and so, dealing in separate segment groups) was not possible. This set of commits should solve that issue.

remark: for some reason (I'm no git expert) this PR also contains the commits from a previous PR already merged, namely #123. If this causes issues, I can refork from main and re-apply these changes.