x86: use PARTUUID instead explicitly specifying the device by default

This changes the x86 image generation to match x86_64, using the PARTUUID for
the rootfs instead of explicitly configuring the device.

It unbreaks KVM with VirtIO, which uses /dev/vda2 instead of /dev/sda2.

Tested in QEMU/KVM with VirtIO, VirtualBox and VMware.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

SVN-Revision: 44966