ppc/spapr, vfio: Turn off MSIX emulation for VFIO devices

ppc/spapr, vfio: Turn off MSIX emulation for VFIO devices

This adds a possibility for the platform to tell VFIO not to emulate MSIX
so MMIO memory regions do not get split into chunks in flatview and
the entire page can be registered as a KVM memory slot and make direct
MMIO access possible for the guest.

This enables the entire MSIX BAR mapping to the guest for the pseries
platform in order to achieve the maximum MMIO preformance for certain
devices.

Tested on:
LSI Logic / Symbios Logic SAS3008 PCI-Express Fusion-MPT SAS-3 (rev 02)

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>