x86/hvm/ioreq: add a new mappable resource type...

x86/hvm/ioreq: add a new mappable resource type...

... XENMEM_resource_ioreq_server

This patch adds support for a new resource type that can be mapped using
the XENMEM_acquire_resource memory op.

If an emulator makes use of this resource type then, instead of mapping
gfns, the IOREQ server will allocate pages which are assigned to the
emulating domain. These pages will never be present in the P2M of the
guest at any point (and are not even shared with the guest) and so are not
vulnerable to any direct attack by the guest.

NOTE: Use of the new resource type is not compatible with use of
      XEN_DMOP_get_ioreq_server_info unless the XEN_DMOP_no_gfns flag is
      set.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>