An ioapic device is already created by the q35 initialization
code, and using "-device ioapic" or "-device kvm-ioapic" will
always fail with "Only 1 ioapics allowed". Remove the
user_creatable flag from the ioapic device classes.

Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-9-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>