s390x/3270: Add emulated terminal3270 device

s390x/3270: Add emulated terminal3270 device

This is a basic implementation of the emulated ccw-attached 3270
called x-terminal3270, which provides visibility of the device in
the qemu monitor and guest. The x prefix indicates that this is
just an experimental implementation for the current stage. This
device will not be compiled until the basic functions are available.

Signed-off-by: Yang Chen <bjcyang@linux.vnet.ibm.com>
Signed-off-by: Jing Liu <liujbjl@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>