Qimpy
Qimpy is a fun, personal Ruby project that wraps the QMP protocol--particularly the parts used by the QEMU Guest Agent--made available by QEMU. This protocol is used to allow the host to commuicate with any guest virtual instances through a Unix Domain Socket using simple, JSON-based messages.
You can find an intro here, and specification details here. However, in its current state, Qimpy only implements wrappers for the parts of QMP related to communicating with the guest agent.
Qimpy & The QEMU Guest Agent
Although Qimpy will likely evolve into a full-blown Ruby gem supporting as many QEMU features as possible, its initial functionality revolves around wrapping the "guest-" series of commands provided by QMP. These include things like read and writing files, changing the number of virtual CPUs, power management, etc.