libxl: remove impossible check for backend != DISK_BACKEND_QDISK

libxl: remove impossible check for backend != DISK_BACKEND_QDISK

In this case we are already in the DISK_BACKEND_QDISK case of a switch
statement on the same variable.

It is possible that we fell through from the DISK_BACKEND_TAP case
(although I'm about to remove that in a subsequent patch), however in
that case we are explicitly falling back from blktap2 to qdisk so
DEVICE_QDISK is still the right answer.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>