target/s390x: simplify gs_allowed()

No need for kvm_enabled() as this function is only called from KVM and
there is no reason why it shouldn't be allowed for tcg. It is simply not
available under tcg.

Also, there is no need to check for the machine type anymore. Just like
ri_enabled(), we can directly use the stored flag, which results in
"true" for the "none" machine.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170818114353.13455-5-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
