New XS_SET_TARGET

New XS_SET_TARGET
Stubdomains (and probably other domain disagregation elements too)
need to be able to tinker with another domain.  This adds
XS_SET_TARGET so that XenStore allows domains to have permissions on
files on which the "target" has permissions.  This also adds
xs_set_target, called by the domain builder when the 'target' option
is used in the configuration.

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