avocado.virt.qemu.machine: Start QEMU process after init

After https://github.com/avocado-framework/avocado/commit/9874c0b5e76aca6e3aaba26ed3a30d198a35aff6, we have to make the QEMU process to start. This patch fixes the problem.

This is a trivial fix I'm publishing to take it off the new version of the vm screenshots feature I'm going to submit.