spapr_iommu: Rename vfio_accel parameter

The vfio_accel parameter used when creating a new TCE table (guest IOMMU
context) has a confusing name.  What it really means is whether we need the
TCE table created to be able to support VFIO devices.

VFIO is relevant, because when available we use in-kernel acceleration of
the TCE table, but that may not work with VFIO devices because updates to
the table are handled in kernel, bypass qemu and so don't hit qemu's
infrastructure for keeping the VFIO host IOMMU state in sync with the guest
IOMMU state.

Rename the parameter to "need_vfio" throughout.  This is a cosmetic change,
with no impact on the logic.

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