block: assert no image modification under BDRV_O_INACTIVE

As long as BDRV_O_INACTIVE is set, the image file is only opened so we
have a file descriptor for it. We're definitely not supposed to modify
the image, it's still owned by the migration source.

This commit is an addition to 09e0c771 but the assert() is added to
bdrv_truncate().

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
Message-id: 1491405505-31620-3-git-send-email-den@openvz.org
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -3328,6 +3328,8 @@ int bdrv_truncate(BdrvChild *child, int64_t offset, Error **errp)
         return -EACCES;
     }
 
+    assert(!(bs->open_flags & BDRV_O_INACTIVE));
+
     ret = drv->bdrv_truncate(bs, offset, errp);
     if (ret == 0) {
         ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);