The libxl driver was setting the backend field of libxl_device_disk
structure to LIBXL_DISK_BACKEND_TAP when the driver element of disk
configuration was not specified.  This needlessly forces the use of
blktap driver, which may not be loaded in dom0

https://bugzilla.redhat.com/show_bug.cgi?id=912488

Ian Campbell suggested that LIBXL_DISK_BACKEND_UNKNOWN is a better
default in this case

https://www.redhat.com/archives/libvir-list/2013-February/msg01126.html