Fix port locking on vswitch backend

Bit of a hack - common.py is reading info about the VIFs from
the disk, not xenstore. However, the network backend is only put
into xenstore and isn't on the disk. This patch puts it on the
disk too, but it's not really an optimal solution.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>