block: BDS options may lack the "driver" option

When BDSs are created by qemu itself (e.g. as filters in block jobs),
they may not have a "driver" option in their options QDict.  When
generating a json:{} filename, however, it must always be present.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190201192935.18394-31-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -5618,6 +5618,12 @@ static bool append_strong_runtime_options(QDict *d, BlockDriverState *bs)
         }
     }
 
+    if (!qdict_haskey(d, "driver")) {
+        /* Drivers created with bdrv_new_open_driver() may not have a
+         * @driver option.  Add it here. */
+        qdict_put_str(d, "driver", bs->drv->format_name);
+    }
+
     return found_any;
 }