fdc: Remove user_creatable flag from sysbus-fdc & SUNW,fdtwo

sysbus-fdc and SUNW,fdtwo devices need IRQs to be wired and mmio
to be mapped, and can't be used with -device. Unset
user_creatable on their device classes.

Cc: John Snow <jsnow@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Cc: Thomas Huth <thuth@redhat.com>
Acked-by: John Snow <jsnow@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-6-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>