Assign SR-IOV is disabled by default on ESXi 5.1 and 5.5. To enable SR-IOV, use the esxcli command or host profiles in 5.1. In vSphere 5.5, it can be done through the vSphere Web Client also.
To enable SR-IOV on a Physical Adapter using the esxcli
command in ESXi 5.1 or 5.5 Please check this KB :
VMware KB: SR-IOV support status FAQ
Best regards
Yours, Oscar