Compute: improve usage of Xen driver support

Commit 69564763960cddc249138469811a0a771db16e19 added a check,
which was called twice, in get_next_device_name.

This is called once now.

Change-Id: I45c918bc64ed8049179fc01a7cd3ad81e4682fe1
Merge "Compute: improve usage of Xen driver support"