spapr_iommu: Provide a function to switch a TCE table to allowing VFIO

spapr_iommu: Provide a function to switch a TCE table to allowing VFIO

Because of the way non-VFIO guest IOMMU operations are KVM accelerated, not
all TCE tables (guest IOMMU contexts) can support VFIO devices.  Currently,
this is decided at creation time.

To support hotplug of VFIO devices, we need to allow a TCE table which
previously didn't allow VFIO devices to be switched so that it can.  This
patch adds an spapr_tce_set_need_vfio() function to do this, by
reallocating the table in userspace if necessary.

Currently this doesn't allow the KVM acceleration to be re-enabled if all
the VFIO devices are removed.  That's an optimization for another time.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>