xenlight: add call to libxl_domain_setmaxmem

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>
Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
xenlight: LIBXL_SHUTDOWN_REASON_UNKNOWN is now 255

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>