[RFC] VM screendump thread (vm videos upcoming)

Create a VM screendump thread (necessary to encode VM videos) on the `Machine()` class level, as in opposition to taking care of it on other parts of the code, such as the `virt-test` approach.

One problem I'm finding here is that some of the screenshots generated by the VMs state  `Guest has not initialized the display (yet)`, something that I never had/noticed before on `virt-test`. I'm still unsure why that is happening [1], and if there's anything clean I can do to get rid of those. Ideas welcome.

[1] My first guess is that we are starting the screendump thread way too early after the QEMU process started, something that never occurred with `virt-test`.

![6d8d2c7c-0001](https://cloud.githubusercontent.com/assets/296807/4734526/e24a08d4-59d4-11e4-911b-5dc2b20c5c14.jpg)