block: Default to enabled write cache in blk_new()

The existing users of the function are:

1. blk_new_open(), which already enabled the write cache
2. Some test cases that don't care about the setting
3. blockdev_init() for empty drives, where the cache mode is overridden
   with the value from the options when a medium is inserted

Therefore, this patch doesn't change the current behaviour. It will be
convenient, however, for additional users of blk_new() (like block
jobs) if the most sensible WCE setting is the default.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>

block/block-backend.c

@@ -125,6 +125,8 @@ BlockBackend *blk_new(void)
 
     blk = g_new0(BlockBackend, 1);
     blk->refcnt = 1;
+    blk_set_enable_write_cache(blk, true);
+
     qemu_co_queue_init(&blk->public.throttled_reqs[0]);
     qemu_co_queue_init(&blk->public.throttled_reqs[1]);
 
@@ -160,7 +162,6 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
         return NULL;
     }
 
-    blk_set_enable_write_cache(blk, true);
     blk->root = bdrv_root_attach_child(bs, "root", &child_root, blk);
 
     return blk;