hw/nvram/spapr_nvram: Device can not be created by the users

Trying to add a spapr-nvram device currently aborts QEMU like this:

$ ppc64-softmmu/qemu-system-ppc64  -device spapr-nvram
qemu-system-ppc64: hw/ppc/spapr_rtas.c:407: spapr_rtas_register:
 Assertion `!rtas_table[token].name' failed.
Aborted (core dumped)

This NVRAM device registers RTAS calls during its realize function
and thus can only be used once - and that's internally from spapr.c.
So let's mark the device with user_creatable = false to avoid that
the users can crash their QEMU this way.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
