S390: Documentation for CCW address type

S390: Documentation for CCW address type

The native bus for s390 I/O is called CCW (channel command word).
As QEMU has added basic support for the CCW bus, i.e. the
ability to assign CCW devnos (bus addresses) to devices.
Domains with the new machine type s390-ccw-virtio can use the
CCW bus. Currently QEMU will only allow to define virtio
devices on the CCW bus.
Here we add the new machine type and the new device address to the
schema definition and add a new paragraph to the domain XML
documentation.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>