s390: sclp ascii console support

s390: sclp ascii console support

This code adds console support  by implementing SCLP's ASCII Console
Data event. This is the same console as LPARs ASCII console or z/VMs
sysascii.

The console can be specified manually with something like
-chardev stdio,id=charconsole0 -device sclpconsole,chardev=charconsole0,id=console0

Newer kernels will autodetect that console and prefer that over virtio
console.

When data is received from the character layer it creates a service
interrupt to trigger a Read Event Data command from the guest that will
pick up the received character byte-stream.
When characters are echo'ed by the linux guest a Write Event Data occurs
which is forwarded by the Event Facility to the console that supports
a corresponding mask value.
Console resizing is not supported.
The character layer byte-stream is buffered using a fixed size iov
buffer.

Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>