[CORE] Improve sanity checks in vmm_host_ram and vmm_host_vapool APIs

Minor improvements in sanity checks of vmm_host_ram_reserve(),
vmm_host_ram_free(), vmm_host_ram_isfree(), vmm_host_vapool_reserve(),
vmm_host_vapool_free(), and vmm_host_vapool_isfree().

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