The driver in the xen config file needs to be changed when dealing with
files rather then bdevs.

This patch does two things:
- Add _GetConfigFileDiskData to XenHypervisor which returns the correct
  disk xen config line. This function checks the logical disk type of
  every given block device, such that also hybrid (e.g. mixed drbd and
  file VBDs) are possible
- Make Xen[Pvm|Hvm]Hypervisor._WriteConfigFile() a classmethod to be
  able to call the helper function _GetConfigFileDiskData() in their
  parent XenHypervisor

Reviewed-by: iustinp