[CORE] Make host_physical_addr optional for alloced_rom & alloced_ram

For alloced_rom and alloced_ram regions we allocate host RAM when
creating the guest aspace hence "host_physical_addr" attribute is
not required for alloced_rom and alloced_ram regions. This patch
makes this thing more explicit in the vmm_guest_aspace implementation.

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