[CORE] Generic plugable load balancer

This patch introduces a generic load balancer which in itself
does no load balancing but we can plug vmm_loadbal_algo instances.

The generic load balancer will select best rated vmm_loadbal_algo
instance and use that for doing load balancing.

People can now write their own proprietary or open source load
balancing algorithm and plug this at runtime.

Please note that built-in load balancing algorithm will have to
be GPL because it will be linked directly with Xvisor binary which
is GPL.

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