log: enable QMI traces when showing debug logs

Use the new `qmi_utils_set_traces_enabled()' to specify that we want QMI traces
when running with DEBUG logs.

Sync with libqmi:

commit 35dcb4bb6ed2755d968cf97d69faff9ed5f6871f
Author: Aleksander Morgado <aleksander@lanedo.com>
Date:   Tue Oct 9 13:44:16 2012 +0200

    libqmi-glib: message traces compiled always

    Message traces have been very useful when debugging issues in the protocol, and
    we should avoid requiring a full recompilation in order to get them enabled.

    Instead, we provide two new API methods, `qmi_utils_(get|set)_traces_enabled()',
    which allow specifying whether traces should be dumped with g_debug() or not.

src/mm-log.c

@@ -27,6 +27,10 @@
 #include <ModemManager.h>
 #include <mm-errors-types.h>
 
+#if defined WITH_QMI
+#include <libqmi-glib.h>
+#endif
+
 #include "mm-log.h"
 
 enum {
@@ -179,9 +183,15 @@ mm_log_set_level (const char *level, GError **error)
             break;
         }
     }
+
     if (!found)
         g_set_error (error, MM_CORE_ERROR, MM_CORE_ERROR_INVALID_ARGS,
                      "Unknown log level '%s'", level);
+
+#if defined WITH_QMI
+    qmi_utils_set_traces_enabled (log_level & LOGL_DEBUG ? TRUE : FALSE);
+#endif
+
     return found;
 }