block: Remove BDRV_O_COPY_ON_READ for bs->file

Copy on Read makes sense on the format level where backing files are
implemented, but it's not required on the protocol level. While it
shouldn't actively break anything to have COR enabled on both layers,
needless serialisation and allocation checks may impact performance.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>

block.c

@@ -793,7 +793,7 @@ static int bdrv_inherited_flags(int flags)
     }
 
     /* Clear flags that only apply to the top layer */
-    flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
+    flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_COPY_ON_READ);
 
     return flags;
 }