Add vsm_get_event_fd and vsm_enter_eventloop

Add vsm_get_event_fd and vsm_enter_eventloop

[Feature]       Provide possibility to implement custom event loop for
                user of vasum client library
[Cause]         Integration with other services
[Solution]      Possibility to fetch event fd and run event loop
[Verification]  run ClientSuite tests

Change-Id: Ic47e5de8bfd09889330d97d35d4c9ef47f238953