[CORE] Use buddy allocator to manage VAPOOL

Currently, we use bitmap to manage VAPOOl hence we have following
runtime complexities:
VAPOOL alloc: O(n)
VAPOOL reserve: O(1)
VAPOOL free: O(1)

From above it is clear the VAPOOL allocations are slower because
of bitmap usage.

This patch introduces use of buddy allocator to manage VAPOOL
and updated runtime complexities are:
VAPOOL alloc: O(log n)
VAPOOL reserve: O(log n)
VAPOOL free: O(log n)

The patch also does few simplifications to vmm_host_aspace_init().

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