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: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/cow.c

@@ -171,14 +171,14 @@ static int cow_update_bitmap(BlockDriverState *bs, int64_t sector_num,
     return error;
 }
 
-static int cow_read(BlockDriverState *bs, int64_t sector_num,
-                    uint8_t *buf, int nb_sectors)
+static int coroutine_fn cow_read(BlockDriverState *bs, int64_t sector_num,
+                                 uint8_t *buf, int nb_sectors)
 {
     BDRVCowState *s = bs->opaque;
     int ret, n;
 
     while (nb_sectors > 0) {
-        if (bdrv_is_allocated(bs, sector_num, nb_sectors, &n)) {
+        if (bdrv_co_is_allocated(bs, sector_num, nb_sectors, &n)) {
             ret = bdrv_pread(bs->file,
                         s->cow_sectors_offset + sector_num * 512,
                         buf, n * 512);