Process init on demand

Move SubProcess() internal subprocess initialization to a separate method, and don't execute it at init time. Add a new .start() method, for people wanting to use background processes, such as QEMU instances inside a virt related test.

With this, we honor the user's expectation for the run() method - start a process and wait until it ends. Also, it requires a minimal change in the virt plugin (calling the .start() method in the QEMU SubProcess() call).
Define a proper documentation, like "Start running a program, useful for background process" (as you say in the PR).