block: use public bdrv_is_allocated() interface

There is no need for bdrv_commit() to use the BlockDriver
.bdrv_is_allocated() interface directly.  Converting to the public
interface gives us the freedom to drop .bdrv_is_allocated() entirely in
favor of a new .bdrv_co_is_allocated() in the future.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -1013,7 +1013,7 @@ int bdrv_commit(BlockDriverState *bs)
     buf = g_malloc(COMMIT_BUF_SECTORS * BDRV_SECTOR_SIZE);
 
     for (sector = 0; sector < total_sectors; sector += n) {
-        if (drv->bdrv_is_allocated(bs, sector, COMMIT_BUF_SECTORS, &n)) {
+        if (bdrv_is_allocated(bs, sector, COMMIT_BUF_SECTORS, &n)) {
 
             if (bdrv_read(bs, sector, buf, n) != 0) {
                 ret = -EIO;