public/xen.h: add a definition for a 'valid domid' mask

public/xen.h: add a definition for a 'valid domid' mask

A subsequent patch will modify libxl to allow selection of a random domid
value when creating domains. Valid values are limited to a width of 15 bits,
so add an appropriate mask definition to the public header.

NOTE: It is reasonable for this mask definition to be in a Xen public header
      rather than in, say, a libxenctrl header since it relates to the
      validity of a value passed to XEN_DOMCTL_createdomain. This new
      definition is placed in xen.h rather than domctl.h only to co-locate
      it with other domid-related defitions.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Julien Grall <julien@xen.org>