allwinner-ahci: Remove user_creatable flag

allwinner-ahci needs its IRQ to be connected and mmio to be
mapped (this is done by the alwinner-a10 device realize method),
and won't work with -device. Remove the user_creatable flag from
the device class.

Cc: John Snow <jsnow@redhat.com>
Cc: qemu-block@nongnu.org
Cc: Beniamino Galvani <b.galvani@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Cc: Marcel Apfelbaum <marcel@redhat.com>
Acked-by: John Snow <jsnow@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-12-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>