cpus hw target: Use warn_report() & friends to report warnings

cpus hw target: Use warn_report() & friends to report warnings

Calling error_report() in a function that takes an Error ** argument
is suspicious.  Convert a few that are actually warnings to
warn_report().

While there, split a warning consisting of multiple sentences to
conform to conventions spelled out in warn_report()'s contract.

Cc: Alex Bennée <alex.bennee@linaro.org>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Fam Zheng <famz@redhat.com>
Cc: Wei Huang <wei@redhat.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20181017082702.5581-5-armbru@redhat.com>