target-s390x: PER storage-alteration event support

target-s390x: PER storage-alteration event support

For the PER storage-alteration event we can use the QEMU watchpoint
infrastructure. When PER is enabled or PER control register changed we
enable the corresponding watchpoints. When a watchpoint arises we can
save the event. Unfortunately the current code does not provide the
address space used to trigger the watchpoint. For now we assume it comes
from the default ASC.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Alexander Graf <agraf@suse.de>