block/gluster: add support to choose libgfapi logfile
currently all the libgfapi logs defaults to '/dev/stderr' as it was hardcoded
in a call to glfs logging api. When the debug level is chosen to DEBUG/TRACE,
gfapi logs will be huge and fill/overflow the console view.
This patch provides a commandline option to mention log file path which helps
in logging to the specified file and also help in persisting the gfapi logs.