s390x/3270: Mark non-migratable and enable the device

s390x/3270: Mark non-migratable and enable the device

Mark 3270 as non-migratable for the experimental stage. Enable
the 3270 device so that we can use x3270 client to operate the guest.

Run qemu with the arguments:
    -chardev socket,id=char3270_0,host=0.0.0.0,port=23,nowait,server,tn3270 \
    -device x-terminal3270,chardev=char3270_0,devno=fe.0.000a,id=terminal3270_0 \

There are some restrictions for the first stage: We don't support SSL
connections, multiple client connections and client resizing. Only
tested with the x3270 client.

Signed-off-by: Jing Liu <liujbjl@linux.vnet.ibm.com>
Signed-off-by: Yang Chen <bjcyang@linux.vnet.ibm.com>
Reviewed-by: QingFeng Hao <haoqf@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>