Fix VimAdminThread run method

The run_coroutine_threadsafe() function is used to schedule a coroutine object from a different thread and returns a concurrent.futures.Future.
run_coroutine_threadsafe is unnecessary to run the main task and replaced with asyncio.run().

Change-Id: I8ea31828a9798140d596165443bdf26659b4eef8
Signed-off-by: Gulsum Atici <gulsum.atici@canonical.com>