netdev-vport: Make netdev_vport_patch_peer() return a malloc()'d string.

netdev-vport: Make netdev_vport_patch_peer() return a malloc()'d string.

When threading comes into the picture there arises the possibility of a
race between netdev_vport_patch_peer()'s caller using the returned string
and another caller changing the peer.  It is safer to return a copy.

Signed-off-by: Ben Pfaff <blp@nicira.com>