xen-hvm: try to use xenforeignmemory_map_resource() to map ioreq pages

Xen 4.11 has a new API to directly map guest resources. Among the resources
that can be mapped using this API are ioreq pages.

This patch modifies QEMU to attempt to use the new API should it exist,
falling back to the previous mechanism if it is unavailable.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>