arm: appliance: Add support for device trees (dtb's).

If supermin-helper >= 4.1.5 is found, use the new-style syntax and if
the architecture requires it (only ARM for now) implement device
trees.

This means we pass a supermin-helper --dtb option to find the right
device tree (currently Versatile Express A9, since that's what we pass
to qemu in the -M option).  This makes supermin-helper find a
compatible device tree file.

Also that we pass the corresponding dtb file to qemu via the qemu -dtb
option, or to libvirt via the <dtb> element.