block: Fix bdrv_is_allocated() return value

bdrv_is_allocated() should return either 0 or 1 in successful cases.
We're lucky that currently, the callers that rely on this (e.g. because
they check for ret == 1) don't seem to break badly. They just might skip
some optimisation or in the case of qemu-io 'map' print separate lines
where a single line would suffice. In theory, a wrong allocation status
could lead to image corruption with certain operations, so let's fix
this quickly.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

block.c

@@ -4040,7 +4040,7 @@ int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t sector_num,
     if (ret < 0) {
         return ret;
     }
-    return (ret & BDRV_BLOCK_ALLOCATED);
+    return !!(ret & BDRV_BLOCK_ALLOCATED);
 }
 
 /*