xive: Add a "presenter" link property to the TCTX object

This will be used in subsequent patches to access the XIVE associated to
a TCTX without reaching out to the machine through qdev_get_machine().

Signed-off-by: Cédric Le Goater <clg@kaod.org>
[ groug: - split patch
         - write subject and changelog ]
Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20200106145645.4539-9-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>