block: Allow bdrv_new() with and without graph lock

bdrv_new() calls bdrv_drained_begin(), which can poll and therefore
can't be called while holding the graph lock. One option to make sure
that this call is allowed would be marking bdrv_new() GRAPH_UNLOCKED.

However, this is actually an unnecessary restriction because we know
that we only just created the BlockDriverState and it isn't even part of
the graph yet. We can use bdrv_do_drained_begin_quiesce() instead to
avoid the polling, which means that bdrv_new() can now safely be called
from callers that hold the graph lock as well as from callers that
don't.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -431,7 +431,7 @@ BlockDriverState *bdrv_new(void)
     bs->block_status_cache = g_new0(BdrvBlockStatusCache, 1);
 
     for (i = 0; i < bdrv_drain_all_count; i++) {
-        bdrv_drained_begin(bs);
+        bdrv_do_drained_begin_quiesce(bs, NULL);
     }
 
     QTAILQ_INSERT_TAIL(&all_bdrv_states, bs, bs_list);