Fix error in admin interface when sorting test jobs
Fix error in admin interface when sorting test jobs

Filtering testjobs by the requested device type needs to
select a queryset based on requested_device_type.

Change-Id: I3e93d679f40ff355ed3872442edbb34a53bc1b3b

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server