bgpd: fix leaving hidden state

bgpd: fix leaving hidden state

Upon configuration of a VRF instance that references an absent default
VRF with "import vrf default", the default instance is created in hidden
state. However, the default instance is not properly un-hidden when
configured.

Restore the behavior prior to commit below.

Fixes: 9f7177af13 ("bgpd: fix duplicate BGP instance created with unified config")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
(cherry picked from commit 70e07678bfe554dd5be30a605ddf6c0fe3a8a39b)
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>