bgp: support add/delete VPNv6 VRF

added VPNv6 route family VRF support.
This patch enables addition and deletion of VPNv6 VRF.

Signed-off-by: Hiroshi Yokoi <yokoi.hiroshi@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bgp: support add/delete vpnv6 prefix

added VPNv6 prefix support.
This patch enables addition and deletion of VPNv6 prefix.

Signed-off-by: Hiroshi Yokoi <yokoi.hiroshi@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>