vfio-pci: Mask INTx if a device is not capabable of enabling it

At the moment VFIO rightfully assumes that INTx is supported if
the interrupt pin is not set to zero in the device config space.
However if that is not the case (the pin is not zero but pdev->irq is),
vfio_intx_enable() fails.

In order to prevent the userspace from trying to enable INTx when we know
that it cannot work, let's mask the PCI_INTERRUPT_PIN register.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>