block: Keep "filename" option after parsing

Currently, bdrv_file_open() always removes the "filename" option from
the options QDict after bdrv_parse_filename() has been (successfully)
called. However, for drivers with bdrv_needs_filename, it makes more
sense for bdrv_parse_filename() to overwrite the "filename" option and
for bdrv_file_open() to fetch the filename from there.

Since there currently are no drivers that implement
bdrv_parse_filename() and have bdrv_needs_filename set, this does not
change current behavior.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -1017,7 +1017,12 @@ static int bdrv_file_open(BlockDriverState *bs, const char *filename,
             ret = -EINVAL;
             goto fail;
         }
-        qdict_del(*options, "filename");
+
+        if (!drv->bdrv_needs_filename) {
+            qdict_del(*options, "filename");
+        } else {
+            filename = qdict_get_str(*options, "filename");
+        }
     }
 
     if (!drv->bdrv_file_open) {