Using isolateDevice limits the video cards that X will touch when doing a pci
analysis. This allows to have different X servers in different video cards, that's
mostly used for multiseat. I wanted to have a Xinerama X (with two video cards)
and a secondary seat X. But isolateDevice would not allow to "isolate" two
video cards, so the Xinerama+multiseat was imposible.
The patch attached changes the xf86IsolateDevice to list of PCIids so it's
posible to have more than one isolated device per X server.