block: Convert .bdrv_truncate callback to coroutine_fn

bdrv_truncate() is an operation that can block (even for a quite long
time, depending on the PreallocMode) in I/O paths that shouldn't block.
Convert it to a coroutine_fn so that we have the infrastructure for
drivers to make their .bdrv_co_truncate implementation asynchronous.

This change could potentially introduce new race conditions because
bdrv_truncate() isn't necessarily executed atomically any more. Whether
this is a problem needs to be evaluated for each block driver that
supports truncate:

* file-posix/win32, gluster, iscsi, nfs, rbd, ssh, sheepdog: The
  protocol drivers are trivially safe because they don't actually yield
  yet, so there is no change in behaviour.

* copy-on-read, crypto, raw-format: Essentially just filter drivers that
  pass the request to a child node, no problem.

* qcow2: The implementation modifies metadata, so it needs to hold
  s->lock to be safe with concurrent I/O requests. In order to avoid
  double locking, this requires pulling the locking out into
  preallocate_co() and using qcow2_write_caches() instead of
  bdrv_flush().

* qed: Does a single header update, this is fine without locking.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

include/block/block.h

@@ -300,8 +300,12 @@ int coroutine_fn bdrv_co_pwrite_zeroes(BdrvChild *child, int64_t offset,
 BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
     const char *backing_file);
 void bdrv_refresh_filename(BlockDriverState *bs);
+
+int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset,
+                                  PreallocMode prealloc, Error **errp);
 int bdrv_truncate(BdrvChild *child, int64_t offset, PreallocMode prealloc,
                   Error **errp);
+
 int64_t bdrv_nb_sectors(BlockDriverState *bs);
 int64_t bdrv_getlength(BlockDriverState *bs);
 int64_t bdrv_get_allocated_file_size(BlockDriverState *bs);

include/block/block_int.h

@@ -289,8 +289,8 @@ struct BlockDriver {
      * bdrv_parse_filename.
      */
     const char *protocol_name;
-    int (*bdrv_truncate)(BlockDriverState *bs, int64_t offset,
-                         PreallocMode prealloc, Error **errp);
+    int coroutine_fn (*bdrv_co_truncate)(BlockDriverState *bs, int64_t offset,
+                                         PreallocMode prealloc, Error **errp);
 
     int64_t (*bdrv_getlength)(BlockDriverState *bs);
     bool has_variable_length;