[CORE] Distinguish behaviour of vmm_host_memunmap() and vmm_host_free_pages()

The vmm_host_memunmap() as-per semantics should free entire alloced/reserved
virtual space whereas vmm_host_free_pages() should only free selected portion
of virtual space.

This patch does minor cosmetic changes to both this APIs so that this meaning
or semantics can be easily inferred from code.

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