S390: Add SCLP console front end support

S390: Add SCLP console front end support

The SCLP console is the native console type for s390 and is preferred
over the virtio console as it doesn't require special drivers and
is more efficient. Recent versions of QEMU come with SCLP support
which is hereby enabled.

The new target types 'sclp' and 'sclplm' can be used to specify a
SCLP console. Adding documentation, domain schema and XML processing
support.

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