igb: don't allow SR-IOV without MSI-X

MSI-X interrupts are required for SR-IOV operation. Check to make sure
they're enabled before allowing the user to turn on VFs.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>