Process init on demand v3

Follow up to PR #241.

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).

Changelog:

https://github.com/lmr/avocado/compare/process_init_on_demand_v2...process_init_on_demand_v3?diff=split

v2 -> v3:

Added functional tests (adapted from original code made by @ruda).