cow: do not call bdrv_co_is_allocated

As we change bdrv_is_allocated to gather more information from bs and
bs->file, it will become a bit slower.  It is still appropriate for online
jobs, but not for reads/writes.  Call the internal function instead.
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/cow.c

@@ -212,7 +212,7 @@ static int coroutine_fn cow_read(BlockDriverState *bs, int64_t sector_num,
     int ret, n;
 
     while (nb_sectors > 0) {
-        if (bdrv_co_is_allocated(bs, sector_num, nb_sectors, &n)) {
+        if (cow_co_is_allocated(bs, sector_num, nb_sectors, &n)) {
             ret = bdrv_pread(bs->file,
                         s->cow_sectors_offset + sector_num * 512,
                         buf, n * 512);