[CORE] vmm_guest_aspace: More improvements to region add/del APIs

This patch does few improvements to region add/del APIs:
1. Rename vmm_guest_aspace_xxx_region() APIs to vmm_guest_xxx_region() APIs
2. Introduce "initialized" flag in struct vmm_guest_aspace to ensure that
   vmm_guest_xxx_region() and other APIs fail when called before doing
   vmm_guest_aspace_init().

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