xen: add missing guest address range checks to XENMEM_exchange handlers

xen: add missing guest address range checks to XENMEM_exchange handlers

Ever since its existence (3.0.3 iirc) the handler for this has been
using non address range checking guest memory accessors (i.e.
the ones prefixed with two underscores) without first range
checking the accessed space (via guest_handle_okay()), allowing
a guest to access and overwrite hypervisor memory.

This is XSA-29 / CVE-2012-5513.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>