Palo, as with any Hypervisor offers the added benefits being able to customize your bandwidth requirements (QoS) on a vNIC by vNIC basis. One of the great value adds for this is a tiered service approach. You can have various uplinks providing distinct QoS levels, and assign VMs to the appropriate Uplinks based on the CoS levels you desire.
Though nothing is written in stone yet, there's a good change VN-Link in HW could be coming to Xenserver in the not so distant future.
Here's a couple of docs you might find useful.
CVD (Cisco Validated Design) for Xenserver & Netapp
UCS & Citrix XenDesktop and Atlantis ILIO
Regards,
Robert