block: push bdrv_change_backing_file error checking up from drivers

This check applies to all drivers, but QED lacks it.
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -1460,6 +1460,11 @@ int bdrv_change_backing_file(BlockDriverState *bs,
 {
     BlockDriver *drv = bs->drv;
 
+    /* Backing file format doesn't make sense without a backing file */
+    if (backing_fmt && !backing_file) {
+        return -EINVAL;
+    }
+
     if (drv->bdrv_change_backing_file != NULL) {
         return drv->bdrv_change_backing_file(bs, backing_file, backing_fmt);
     } else {

block/qcow2.c

@@ -1011,11 +1011,6 @@ fail:
 static int qcow2_change_backing_file(BlockDriverState *bs,
     const char *backing_file, const char *backing_fmt)
 {
-    /* Backing file format doesn't make sense without a backing file */
-    if (backing_fmt && !backing_file) {
-        return -EINVAL;
-    }
-
     pstrcpy(bs->backing_file, sizeof(bs->backing_file), backing_file ?: "");
     pstrcpy(bs->backing_format, sizeof(bs->backing_format), backing_fmt ?: "");