[TESTS] Add DTS files for various ARM guest types

With the new "vfs fdt_load" command we can create device tree
for guest instance form FDT file (i.e. DTB file) hence this
patch adds DTS files for all types of ARM guests and also
updates Xvisor disk image creation to include guest layout
DTB file.

Signed-off-by: Anup Patel <anup@brainfault.org>