Generic PCI passthrough support

[arighi]

In #165 we have introduced some basic support for NVIDIA GPU passthrough. However, as pointed out by @SPYFF, it would be nice to generalize this feature and provide a more versatile interface to support PCI passthrough with any device.

Therefore, I'm opening this issue to start collecting ideas, suggestions, etc. especially on how we want to design the virtmg-ng interface.

[SPYFF]

Thanks for moving it to a separate issue. Hopefully people will come forward and confirm whether or not it can be done in a generic way. Most of the commands, blogposts and writeups on VFIO passthrough are almost a decade old, the most recent ones I have seen just reuse those commands. My guess is that there have been some clever tricks since then that I'm not aware of. I may be wrong, but I still see people posting ifconfig and brctl configuration tips online. :-)