s390x/sclp: Mark the sclp device with user_creatable = false

s390x/sclp: Mark the sclp device with user_creatable = false

The "sclp" device is just an internal device that can not be instantiated
by the users. If they try to use it, they only get a simple error message:

$ qemu-system-s390x -nographic -device sclp
qemu-system-s390x: Option '-device s390-sclp-event-facility' cannot be
handled by this machine

Since sclp_init() tries to create a TYPE_SCLP_EVENT_FACILITY which is
a non-pluggable sysbus device, there is really no way that the "sclp"
device can be used by the user, so let's set the user_creatable = false
accordingly.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1507125199-22562-1-git-send-email-thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Reviewed-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Acked-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>