Coverity points out (CID 1513106, 1513107) that MemoryListener is a
192 byte struct which we are passing around by value.  Switch to
passing a const pointer into xen_register_ioreq() and then to
xen_do_ioreq_register().  We can also make the file-scope
MemoryListener variables const, since nothing changes them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230718101057.1110979-1-peter.maydell@linaro.org>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>