xenconsoled: use grant references instead of map_foreign_range

xenconsoled: use grant references instead of map_foreign_range

Grant references for the xenstore and xenconsole shared pages exist, but
currently only xenstore uses these references.  Change the xenconsole
daemon to prefer using the grant reference over map_foreign_range when
mapping the shared console ring.

This allows xenconsoled to be run in a domain other than dom0 if set up
correctly - for libxl, the xenstore path /tool/xenconsoled/domid
specifies the domain containing xenconsoled.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>