rework PCI capabilities

[tmakatos]

Currently handling of standard PCI capabilities is left to the device implementation, however that shouldn't be the case as it's the same for all kinds of devices.

[tmakatos]

Ideally we'd like to have a flexible array member at the end of lm_dev_info_t for each capability, where each element of the array would be a union of all possible capability types. However I don't think this is possible as there can be vendor-specific capabillities of unknown size. Therefore the more realistic option would be to introduce a function e.g. lm_pci_cap_add that adds a capability to an existing context. @franciozzy thoughts?

[tmakatos]

Actually I think the current implementation can work, either way the user will be maintaining a pointer to their capability, so all we need to do is to replace the fn field with a buf field in lm_cap_t.

[tmakatos]

I'm looking at the PCI specification and I'm not sure how much of it we can handle in the library we need to go through the spec very thoroughly.

[tmakatos]

I've moved handling of PCI capabilities into MUSER in my vfio-over-socket branch. Capabilities are still maintained in a list which I think need to be replaced by keeping them in a buffer.

[tmakatos]

fixed in vfio-user branch