spapr: Clean up misuse of qdev_init() in xics-kvm creation

We call try_create_xics() to create a "xics-kvm".  If it fails, we
call it again to fall back to plain "xics".

try_create_xics() uses qdev_init().  qdev_init()'s error handling has
an unwanted side effect: it calls qerror_report_err(), which prints to
stderr.  Looks like an error, but isn't.

In QMP context, it would stash the error in the monitor instead,
making the QMP command fail.  Fortunately, it's only called from board
initialization, never in QMP context.

Clean up by cutting out the qdev_init() middle-man: set property
"realized" directly.

While there, improve the error message when we can't satisfy an
explicit user request for "xics-kvm", and exit(1) instead of abort().
Simplify the abort when we can't create "xics".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
[agraf: squash in fix for uninitialized variable from mdroth]
Signed-off-by: Alexander Graf <agraf@suse.de>