gluster: fix .bdrv_reopen_prepare when backing file is a JSON object

When the backing_file is specified as a JSON object, the
qemu_gluster_reopen_prepare() fails with this message:
    invalid URI json:{"server.0.host": ...}

In this case, we should call qemu_gluster_init() using the QDict
'state->options' that contains the JSON parameters already parsed.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1542445
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20190715132844.506584-1-sgarzare@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/gluster.c

@@ -931,7 +931,17 @@ static int qemu_gluster_reopen_prepare(BDRVReopenState *state,
     gconf->has_debug = true;
     gconf->logfile = g_strdup(s->logfile);
     gconf->has_logfile = true;
-    reop_s->glfs = qemu_gluster_init(gconf, state->bs->filename, NULL, errp);
+
+    /*
+     * If 'state->bs->exact_filename' is empty, 'state->options' should contain
+     * the JSON parameters already parsed.
+     */
+    if (state->bs->exact_filename[0] != '\0') {
+        reop_s->glfs = qemu_gluster_init(gconf, state->bs->exact_filename, NULL,
+                                         errp);
+    } else {
+        reop_s->glfs = qemu_gluster_init(gconf, NULL, state->options, errp);
+    }
     if (reop_s->glfs == NULL) {
         ret = -errno;
         goto exit;