Add support for Qemu devices on cloud instances (gui)

This adds support to the server for running Qemu devices on cloud instances.  This can be used to test the PR in the gns3-server repository.

Right now this branch will git clone and checkout gns3-server branch gns-125, so that it could be tested.  When the server PR is merged, update this PR to checkout gns3-server's dev branch.

You can find small (23MB) qemu images of Microcore linux on the gns community forum. See the Appliances link on the Downloads page. These are handy for testing with.