The VFIO driver routes LSI interrupts by capturing, masking, and then
delivering.  When passing though Mellanox adapters from host to guest,
interrupt storm are reported from host and guest.  That's because the PCI
command register INTx Disable bit doesn't work on Mellanox devices.

  # lspci | grep Mellanox
  0001:05:00.0 Ethernet controller: Mellanox Technologies MT27500 Family [ConnectX-3]
  0005:01:00.0 Ethernet controller: Mellanox Technologies MT26448 [ConnectX EN 10GigE, PCIe 2.0 5GT/s] (rev b0)

Amir Vadai confirmed that all Mellanox devices have same problem.
The patch marks broken INTx masking for all Mellanox adapters.
Suggested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-By: Amir Vadai <amirv@mellanox.com>