[Libguestfs] fallback to virtio_blk if guest kernel lacks virtio_scsi support

Currently virtio_scsi is forced if qemu in the host supports this feature. This test does however not take the capabilities of the started guest into account. As a result no disks will be found if the guest kernel is older than 3.4. I forced qemu_supports_virtio_scsi to return always 0, which seem to work well for the kernel versions as shipped in SLES11SP2, openSuSE 12.1 and older. Is there any functionality that would be missing if a guest kernel has just virtio_blk? How can I force the host tools to not enable virtio_scsi? Should it be a runtime option, or should it be done at build time with --disable-virtio_scsi or something like that? Olaf