hw/isa/pc87312: Mark the device with user_creatable = false

QEMU currently aborts if you try to use the device at the command
line:

$ ppc64-softmmu/qemu-system-ppc64 -S -machine prep -device pc87312
Unexpected error in qemu_chr_fe_init() at chardev/char-fe.c:222:
qemu-system-ppc64: -device pc87312: Device 'parallel0' is in use
Aborted (core dumped)

It uses parallel_hds in its realize function, so I can not be
instantiated by the user again.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
