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: NPaul Durrant <paul.durrant@citrix.com>
Reviewed-by: NAnthony PERARD <anthony.perard@citrix.com>
Signed-off-by: NStefano Stabellini <sstabellini@kernel.org>