bgpd: best path calculation uses peer from imported prefixes

bgpd: best path calculation uses peer from imported prefixes

Until now, when calculating the bgp bestpath route, the peer
comparison could not be performed for imported prefixes, as
the peer origin could not be retrieved. As consequence, the
reason why a given prefix as chosen was wrong: "Locally
configured route" was the main reason, whereas the prefix
was imported from a remote peer.

Fix this by searching for the real peer.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>