cow: use bdrv_co_is_allocated() Now that bdrv_co_is_allocated() is available we can use it instead of the synchronous bdrv_is_allocated() interface. This is a follow-up that Kevin Wolf <kwolf@redhat.com> pointed out after applying the series that introduces bdrv_co_is_allocated(). It is safe to make cow_read() a coroutine_fn because its only caller is a coroutine_fn. Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>