pci: rename virPCIDeviceGetVFIOGroupDev to virPCIDeviceGetIOMMUGroupDev

pci: rename virPCIDeviceGetVFIOGroupDev to virPCIDeviceGetIOMMUGroupDev

I realized after the fact that it's probably better in the long run to
give this function a name that matches the name of the link used in
sysfs to hold the group (iommu_group).

I'm changing it now because I'm about to add several more functions
that deal with iommu groups.