I've been looking for the QEMU Guest Agent for Ubuntu 12.04 LTS. It seems like the Guest Agent is included in the official Repository from Ubuntu 14.04 and up (http://packages.ubuntu.com/trusty/qemu-guest-agent).
Is there a way to get the Guest Agent running in 12.04?
Update Compiling and/or installing qemu-guest-agent from the Trusty Repos seems to be the solution. While testing different VMs I noticed that the hosts have different OS versions (one with Precise/KVM and the other with Trusty/Spice). So my problem seems to be related to the combination of host and guest OSes. I have opened another question for this!