Comment 4 for bug 1729584

Doing some more debugging, it looks like the disk_info['mapping'] is just processed in order:

disk_info={'disk_bus': u'scsi', 'cdrom_bus': 'ide', 'mapping': {u'/dev/sdb': {'bus': u'scsi', 'type': u'disk', 'dev': u'sdb'}, 'root': {'bus': u'scsi', 'boot_index': '1', 'type': u'disk', 'dev': u'sda'}, u'/dev/sda': {'bus': u'scsi', 'boot_index': '1', 'type': u'disk', 'dev': u'sda'}}}

I guess in _get_guest_storage_config this takes place, or maybe in _get_guest_disk_config, but since it seems to just iterate through the mapping in the order provided while incrementing the LUN each time, we end up with this situation.