cpu_s390: Don't check match attribute for host-model CPUs

cpu_s390: Don't check match attribute for host-model CPUs

The match attribute is only relevant for custom mode CPUs. Reporting
failure when match == 'minimum' regardless on CPU mode can cause
unexpected failures. We should only report the error for custom CPUs. In
fact, calling virCPUs390Update on a custom mode CPU should always report
an error as optional features are not supported on s390 either.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>