[Libguestfs] [PATCH] Add virtio-scsi support to febootstrap.

This patch is the minimum possible to add virtio-scsi support to febootstrap. Problem: SCSI devices are detected asynchronously after the Linux kernel module is loaded. The usual solutions to this involve scsi_wait_scan (deprecated apparently) or udev. However febootstrap doesn't use udev, and adding it is a ton of work with a very uncertain outcome. So the minimum possible change is to have libguestfs pass in the appliance device on the Linux command line (eg. root=/dev/sdb) and then busy-wait until it becomes available in the initramfs. Rich.