activate serial console on CentOS 6 (or RedHat EL 6)

How to activate the serial console in CentOS 6. Useful for logging into a virtual guest via 'virsh console'.

/etc/init/ttyS0.conf:

# ttyS0 - agetty

stop on runlevel [016]
start on runlevel [345]

instance ttyS0
respawn
pre-start exec /sbin/securetty ttyS0
exec /sbin/agetty /dev/ttyS0 115200 vt100-nav

then run
start ttyS0