s390: Report AEF features to sysfs

Display various information about the Arm execution Facility (AEF) to
userspace. The files are located in /sys/firmware/aef/. Show if the
feature is available and usable for the host, which formats are
available, and how many arm vCPUS per guest are supported by the
machine. Additionally, provide the raw binary output of QAAF fc1 (Query
Model Capabilities) in the file qmc and fc2 (Initial
System-Registers-Save Area) in the file save_area.

Co-developed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>