[Libguestfs] [PATCH] appliance: Pass root=UUID=... to supermin.

By passing root=UUID=... to supermin, we make the appliance boot process less sensitive to the non-deterministic process of scanning SCSI disks (of which much more to come). This patch should be tested alongside the supermin patch posted here: https://www.redhat.com/archives/libguestfs/2017-April/msg00174.html which in turn requires this supermin patch series: https://www.redhat.com/archives/libguestfs/2017-April/msg00162.html This will require that libguestfs & supermin are upgraded in step in all downstream distributions, and will also invalidate all binary appliances (although we don't ship a binary appliance for 1.37 so that's not a problem). Rich.