block: Add PreallocMode to BD.bdrv_truncate()

Add a PreallocMode parameter to the bdrv_truncate() function implemented
by each block driver. Currently, we always pass PREALLOC_MODE_OFF and no
driver accepts anything else.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170613202107.10125-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/block/rbd.c b/block/rbd.c
index 9da02cd..9151d5c 100644
--- a/block/rbd.c
+++ b/block/rbd.c
@@ -936,11 +936,18 @@
     return info.size;
 }
 
-static int qemu_rbd_truncate(BlockDriverState *bs, int64_t offset, Error **errp)
+static int qemu_rbd_truncate(BlockDriverState *bs, int64_t offset,
+                             PreallocMode prealloc, Error **errp)
 {
     BDRVRBDState *s = bs->opaque;
     int r;
 
+    if (prealloc != PREALLOC_MODE_OFF) {
+        error_setg(errp, "Unsupported preallocation mode '%s'",
+                   PreallocMode_lookup[prealloc]);
+        return -ENOTSUP;
+    }
+
     r = rbd_resize(s->image, offset);
     if (r < 0) {
         error_setg_errno(errp, -r, "Failed to resize file");