Introduce virPolkitAgentCreate and virPolkitAgentDestroy

virPolkitAgentCreate will run the polkit pkttyagent image as an asynchronous
command in order to handle the local agent authentication via stdin/stdout.
The code makes use of the pkttyagent --notify-fd mechanism to let it know
when the agent is successfully registered.

virPolkitAgentDestroy will close the command effectively reaping our
child process