cow: use bdrv_co_is_allocated()

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>