These assorted patches end up with adding virtio-scsi support to
libguestfs.
It passes libguestfs-test-tool, but I haven't yet tried to run the
full set of tests.
In theory > 26 devices can be added, but it's likely that certain
parts of the daemon will break if you actually try this. This of
course needs to be fixed.
Thanks Paolo Bonzini for invaluable help.
Rich.