netback: fixes for netback

This patch contains the following fixes for netback:

• Only unbind the evtchn if it has been bound.
• Set xnb->bridge to NULL after free to prevent double-freeing it.
• Set the MAC address for the host-facing interface to a dummy value.

Sponsored by: Citrix Systems R&D

dev/xen/netback/netback.c:

• Prevent trying to unbind if the evtchn has not been bounded.
• Prevent double-freeing xnb->bridge.
• Set the MAC address of the host-facing interface to a dummy value, so it can work when the interface is added to a bridge.