Thanx for posting your question.
Always love to see interesting SR-IOV questions!
I will go and double check this with my experts, however I am pretty sure you cannot perform any kind of network boot over a SR-IOV interface. The network boot (PXE, iSCSI Boot & FCoE Boot) all use the option ROM for the booting. A SR-IOV VF does not have access to the option ROM.
Again, I will double check this and get back to you, however I think what you need to do is have an emulated device assigned to the VM that can do the PXE boot and then once the VM OS is running, switch over to SR-IOV.
I'll post back more information when I get it.
thanx,
Patrick