vtysh: stop reading config file if user `exit`s from root level.

vtysh: stop reading config file if user `exit`s from root level.

This is required to make sure that we properly send the
XFRR_end_configuration tag to the daemons. Previously if the user had an
`exit` at the root level the parser would just drop out of the config
node and so XFRR_end_configuration, even if sent, would be ignored

Signed-off-by: Christian Hopps <chopps@labn.net>