s390/mm: move translation-exception identification structure to fault.h

Move translation-exception identification structure to new fault.h
header file, change it to a union, and change existing kvm code
accordingly. The new union will be used by subsequent patches.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>