libvirt: add serial ports config

This is the first patch for exposing serial ports on openstack
instances. It contains setting up the instance XML config that creates a
TCP chardevice in libvirt.

Author: Ian Wells
Co-autherd by: Vladan Popovic, Sushma Korati

partialy implements blueprint serial-ports

Change-Id: Ia77909dffe6b48be7e6642cbe8bbab27fb6a627d
Merge "libvirt: add serial ports config"