[IIO] bugfixes related to errors from yocto run

[IIO] bugfixes related to errors from yocto run

1. set base src as async, resolves the error of hanging
  without setting base_src as async, _start_complete gets called twice
2. initializing log_file to NULL
3. NULL check for variable before using it for remove/rmdir
4. remove _query function, was incomplete, and complete function
  was same as parent class function

resolve #1353

Signed-off-by: Parichay Kapoor <pk.kapoor@samsung.com>