fix bug due to using hashmap for fdt
fix bug due to using hashmap for fdt

Root block device was not the first one being appended
to the FDT and so there was some randomness when choosing
from which device to boot.

Signed-off-by: Diana Popa <4240422768c1b315e51818a074064501037757cd@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker