Intel document 333717-008, "Intel® Ethernet Controller X550
Specification Update", version 2.7, dated June 2020, includes errata
#22, added in version 2.1, May 2016, indicating X550 NICs suffer from
the same implementation deficiency as the 700-series NICs:

"The Interrupt Status bit in the Status register of the PCIe
 configuration space is not implemented and is not set as described
 in the PCIe specification."

Without the interrupt status bit, vfio-pci cannot determine when
these devices signal INTx.  They are therefore added to the nointx
quirk.

Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>