[CORE] Build vmm_vserial as module

There is no reason why we cannot disable vmm_vserial or build it as
loadable .xo module.

This patch modularizes vmm_vserial and moves it to core/vio directory
from core/ directory.

In future, we will add virtual input (i.e. virtual keyboard or mouse)
and virtual display frameworks to core/vio directory. This directory
is meant to hold Xvisor centric virtual IO frameworks.

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