[XEND] Fix for creating VLANs using the Xen-API

[XEND] Fix for creating VLANs using the Xen-API

Recent changes to the setup of the peth0 interface have resulted in a
change of its MAC address. Previously it seems to have had a MAC
address of 'fe:ff:ff:ff:ff:ff', but now it has the same MAC address as
eth0. As a consequence to this the Xen-API code to create VLANs
(PIF.create_VLAN("peth0",...)) does not work anymore, since peth0 can
not be identified according to this previous criteria of fake MAC
address. This patch fixes this issue by identifying it through the
prefix 'peth'.

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>