block/gluster: add support for selecting debug logging level

block/gluster: add support for selecting debug logging level

This adds commandline support for the logging level of the
gluster protocol driver, output to stdout.  The option is 'debug',
e.g.:

-drive filename=gluster://192.168.15.180/gv2/test.qcow2,debug=9

Debug levels are 0-9, with 9 being the most verbose, and 0 representing
no debugging output.  The default is the same as it was before, which
is a level of 4.  The current logging levels defined in the gluster
source are:

    0 - None
    1 - Emergency
    2 - Alert
    3 - Critical
    4 - Error
    5 - Warning
    6 - Notice
    7 - Info
    8 - Debug
    9 - Trace

(From: glusterfs/logging.h)

Reviewed-by: Niels de Vos <ndevos@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>