kvm tools: Add interval red-black tree helper

Interval rb-tree allows to directly store interval ranges
and quickly lookup an overlap with a single point or a range.

The helper is based on the kernel rb-tree implementation
(located in <linux/rbtree.h>) which alows for the augmention
of the classical rb-tree to be used as an interval tree.

Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>