Added qemu guest agent support for qemu/kvm

Currently nova doesn't support qemu-guest-agent or other agent like it
for kvm instance, if it is supported then we can get guest info on the
compute node through libvirt qemu extension api(qemuAgentCommand)
through guest agent, I think this is a useful way to help us to
interact with guest more directly and efficiently.
DocImpact: please refer the page of bp

Implements blueprint qemu-guest-agent-support
Change-Id: Ief70907add9ccd425a1594ab06c119256bdc89eb
Merge "Added qemu guest agent support for qemu/kvm"