Commit fa1f57421e0b ("xen/virtio: Enable restricted memory access using
Xen grant mappings") introduced a new requirement for using virtio
devices: the backend now needs to support the VIRTIO_F_ACCESS_PLATFORM
feature.
This is an undue requirement for non-PV guests, as those can be operated
with existing backends without any problem, as long as those backends
are running in dom0.
Per default allow virtio devices without grant support for non-PV
guests.
The setting can be overridden by using the new "xen_virtio_grant"
command line parameter.
Add a new config item to always force use of grants for virtio.
Fixes: fa1f57421e0b ("xen/virtio: Enable restricted memory access using Xen grant mappings")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>