Howto rescan NICs on XenServer

1. First list the physical NIC's:
#xe pif-list

2. After making a note of the information, run the following command for each one of the new NIC's that appear as disconnected:
#xe pif-forget uuid=UUID


3. Next, have the system scan for the correct uuids, you will need the uuid of your server (find it with "xe host-list" ):
#xe pif-scan host-uuid=UUID

4. List the NICs again to see what we have found:
#xe pif-list

5. You should see the correct UUID's of your NICs. 
Then, run the following command for each one of the new NIC's to add them back:
#xe pif-plug UUID

At this point your new NICs should appear as connected.