xenstored: use domain_is_unprivileged instead of checking conn->id

xenstored: use domain_is_unprivileged instead of checking conn->id

This centralizes all the permission checking for privileged domains in
preparation for allowing domains other than dom0 to be privileged.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>