s390x/s390-skeys: Mark the storage key devices with user_creatable = false

s390x/s390-skeys: Mark the storage key devices with user_creatable = false

QEMU currently aborts if the user tries to create a skey device:

$ s390x-softmmu/qemu-system-s390x -nographic -device s390-skeys-qemu
qemu-system-s390x: hw/s390x/s390-skeys.c:30: s390_get_skeys_device:
 Assertion `ss' failed.
Aborted (core dumped)

The storage key devices are only meant to be instantiated one time,
internally. They can not be used by the user, so mark them with
user_creatable = false.

Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <1503569328[email protected]>
Reviewed-by: Claudio Imbrenda <[email protected]>
Reviewed-by: Halil Pasic <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>