[CORE] Add endianness attribute for Guest instance

In virtualization, we typically have same Guest and Host endianness
but there is a going interest running Big-Endian Guest on Little-Endian
Host and vice-versa.

This patch add an optional "endianness" attribute to Guest node and
"is_big_endian" boolean flag to Guest instance.

Very shortly we will be using "is_big_endian" boolean flag in VirtIO
emulators to ensure that big-endian Guest will see big-endian VirtIO
queues and little-endian Guest will see little-endian VirtIO queues
irrespective to underlying host endianness.

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