KVM code will be moved to plugin folder

[Edison Su]

As kvm code depends on libvirt-java, which is incompatible with Apache license. 
I want to move it to plugin folder as we already did for other hypervisors, and 
add a compile option to turn on/off KVM compilation. By default, it's turned 
off. 
If you have any patches against agent/kvm code, please check them in ASAP. I 
want to start the moving in Friday.
Any comments?