s390x: lib: Add QUI getter

Some tests need the information provided by the QUI UVC and lib/s390x/uv.c
already has cached the qui result. Let's add a function to avoid
unnecessary QUI UVCs.

Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>