[ARM] Enable RBD driver for all ARM host

Gradually we move towards loading guest images from disk. There
would be many ARM host not having disk access hence for such
ARM host we will use RAM backed block device as disk containing
guest images.

Due to above, we enable RBD driver by default for all ARM hosts
via their defconfigs.

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