xenserver: Rename network devices to match MAC addresses of physical PIFs.

xenserver: Rename network devices to match MAC addresses of physical PIFs.

XenServer does not rely on Linux to keep the naming of network devices
stable from one boot to the next.  Instead, it requires
interface-reconfigure to ensure that network devices are named such that
they have the MAC address specified for the corresponding physical PIF
in the xapi database.

At one point, we fulfilled this requirement by calling out to the Centos
ifup/ifdown scripts, which rename netdevs as necessary to match the
"HWADDR=" lines in /etc/sysconfig/network-scripts/ifcfg-<devname>.  When
we rewrote interface-reconfigure not to use those scripts, however, we
accidentally dropped that support.  This commit adds back in that renaming.

Bug NIC-20.