[libvirt-users] Understanding and converting between type='drive' and type='pci'

First, if this isn't the right list please tell me where i should post. I've searched for an explanation and I'm either searching for the wrong thing or it's not there. I'm trying to get an understanding of the "controller='0' bus='0' target='0' unit='0' " parameters for 'drive' compared to the "domain='0x0000' bus='0x00' slot='0x05' function='0x0' " parameters for 'pci'. It appears that 'dev hda' to 'dev hdd' is the maximum associated with 'drive', how many are available with 'pci' and what are the valid ranges for those parameters? If I have an xml using 'drive' and want to convert it to 'pci' what do I need to do? I've had trouble with this when moving disk images between hypervisors but have never had the time to investigate fully, does it have to do with how the drives are referenced (/dev/disk/by0id or /dev/[h|s|v]da as opposed to UUID)? Thanks for your help.