block: Make auto-read-only=on default for -drive

While we want machine interfaces like -blockdev and QMP blockdev-add to
add as little auto-detection as possible so that management tools are
explicit about their needs, -drive is a convenience option for human
users. Enabling auto-read-only=on by default there enables users to use
read-only images for read-only guest devices without having to specify
read-only=on explicitly. If they try to attach the image to a read-write
device, they will still get an error message.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

blockdev.c

@@ -590,6 +590,7 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
         qdict_set_default_str(bs_opts, BDRV_OPT_CACHE_NO_FLUSH, "off");
         qdict_set_default_str(bs_opts, BDRV_OPT_READ_ONLY,
                               read_only ? "on" : "off");
+        qdict_set_default_str(bs_opts, BDRV_OPT_AUTO_READ_ONLY, "on");
         assert((bdrv_flags & BDRV_O_CACHE_MASK) == 0);
 
         if (runstate_check(RUN_STATE_INMIGRATE)) {