Many thanks for your feedback. I forgot about Magic SysReq and this
helped me a lot to resolve this issue:
1. First I set sysctl kernel.sysrq=1 to enable alt+sysrq
2. I've set kernel log level to 9 using alt+sysrq+9
3. Poweroff was way more verbose now. Last line on screen was:
Preparing to enter system sleep state S5
Using google I found this:
https://bbs.archlinux.org/viewtopic.php?pid=1014214#p1014214
Resume by PCI-E was enabled in bios. After disabling it my system can
power off again.
Thanks!
Łukasz
