qemu: conf: add option for tuning debug logging level

qemu: conf: add option for tuning debug logging level

This helps in selecting log level of the gluster gfapi, output to stderr.
The option is 'gluster_debug_level', can be tuned by editing
'/etc/libvirt/qemu.conf'

Debug levels ranges 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 gfapi are:

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

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>