virtio-mmio: Remove user_creatable flag

virtio-mmio needs to be wired and mapped by other device or board
code, and won't work with -device. Remove the user_creatable flag
from the device class.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Shannon Zhao <zhaoshenglong@huawei.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-20-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>