Here is a short checklist to make CloudStack 3/4 work with KVM on CentOS 6.3:
1. check hostname is okay
2. disable selinux (or set it in permissive mode)
3. remove qemu-kvm if installed
4. add EPEL repository
5. adjust /etc/cgconfig.conf file by adding:
group virt { cpu { cpu.shares = 9216; } }
6. update centos
7. make sure primary/secondary storages could be mounted
8. reboot
9. install cloudstack agent
10. configure bridge devices
11. connect host from the management server