i386/xen: implement HYPERVISOR_memory_op

Specifically XENMEM_add_to_physmap with space XENMAPSPACE_shared_info to
allow the guest to set its shared_info page.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
[dwmw2: Use the xen_overlay device, add compat support]
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
3 files changed