Fix VIF hotplug scripts following multi-xenopsd support

The script needs to find the vif metadata, which is stored
in a different directory for each xenopsd backend. We plumb through
the name of the backend so the script can find it.

With this change, the interface is properly brought up and
port-locking rules activated.

Fixes #18

Signed-off-by: David Scott <dave.scott@eu.citrix.com>