qga: map GLib log levels to system levels

qga: map GLib log levels to system levels

This patch translates GLib-specific log levels to system ones, so that
they may be used by both *nix syslog() (as a "priority" argument) and
Windows ReportEvent() (as a "wType" argument).

Currently the only codepath to write to "syslog" domain is slog()
function.  However, this patch allows the interface to be extended.

Note that since slog() is using G_LOG_LEVEL_INFO level, its behaviour
doesn't change.

Originally-by: Yuri Pudgorodskiy <yur@virtuozzo.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Tested-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>