lib: s390x: add support for SCLP console read

Add a basic implementation for reading from the SCLP ASCII console. The goal of
this is to support migration tests on s390x. To know when the migration has been
finished, we need to listen for a newline on our console.

Hence, this implementation is focused on the SCLP ASCII console of QEMU and
currently won't work under e.g. LPAR.

Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>