Do not set root device for libvirt+Xen

Currently we are setting the root device for Xen when using libvirt.
When a user is booting an image that contains a full partitioned disk it
fails, since the root device will be passed as "root=/dev/whatever" as
the cmdline to the booting kernel.

Closes-Bug: 1253626
Change-Id: Ia5e3ee8891759c68ee51b219d1a7f80d0afbeffc
Merge "Do not set root device for libvirt+Xen"