s390x/cpumodel: Fix query-cpu-definitions error API violations

s390x/cpumodel: Fix query-cpu-definitions error API violations

qmp_query_cpu_definitions() passes @errp to get_max_cpu_model(), then
frees any error it gets back.  This effectively ignores errors.
Dereferencing @errp is wrong; see the big comment in error.h.  Passing
@errp is also wrong, because it works only as long as @errp is neither
@error_fatal nor @error_abort.  Introduced in commit 38cba1f4d8
"s390x: return unavailable features via query-cpu-definitions".

No caller actually passes such @errp values.

Fix anyway: simply pass NULL to get_max_cpu_model().

Cc: David Hildenbrand <david@redhat.com>
Cc: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20191204093625.14836-16-armbru@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>