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: NJing Liu <liujbjl@linux.vnet.ibm.com>
Signed-off-by: NYang Chen <bjcyang@linux.vnet.ibm.com>
Reviewed-by: NQingFeng Hao <haoqf@linux.vnet.ibm.com>
Reviewed-by: NDong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>