util: Introduce API's for Polkit text authentication

util: Introduce API's for Polkit text authentication

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