hw/ide/microdrive: Mark the dscm1xxxx device with user_creatable = false

QEMU currently aborts with an assertion message when the user is trying
to remove a dscm1xxxx again:

$ aarch64-softmmu/qemu-system-aarch64 -S -M integratorcp -nographic
QEMU 2.9.93 monitor - type 'help' for more information
(qemu) device_add dscm1xxxx,id=xyz
(qemu) device_del xyz
**
ERROR:qemu/qdev-monitor.c:872:qdev_unplug: assertion failed: (hotplug_ctrl)
Aborted (core dumped)

Looks like this device has to be wired up in code and is not meant
to be hot-pluggable, so let's mark it with user_creatable = false.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1503543783-17192-1-git-send-email-thuth@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>