This is the xen part of "pc & q35: Add new machine opt max-ram-below-4g"

Note: this machine option cannot be used to increase the amount
of ram below 4G.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>