New XEN_DOMCTL_set_target

New XEN_DOMCTL_set_target
Stubdomains (and probably other domain disagregation elements too)
need to be able to tinker with another domain.  This adds IS_PRIV_FOR
that extends IS_PRIV by allowing domains to have privileges over a
given "target" domain.  XEN_DOMCTL_set_target permits to set this
"target". A new 'target' configuration option makes the domain builder
use it.

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>