[CORE] Make CONFIG_VAPOOL_SIZE_MB arch specific

The VAPOOL size depends on the underlying arch and cannot be generic.

For ARM32, we only have 12MB of reserved virtual address space not used by Linux guest starting at 0xFF000000. Due to this, our VAPOOL start (or Text base) has to 0xFF000000 and maximum size of VAPOOL is 12MB.

For ARM32VE, ARM64, and x86_64 there is no constraint on VAPOOL size because Xvisor virtual address space is totally isolated from guest address space.

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