s390/zdtm: Add s390x_reg_check test case

This test can be used to verify FP and VX registers on s390:

- Verify that "criu restore" sets the correct register sets
  from "criu dump":
  $ zdtmp.py run -t zdtm/static/s390x_regs_check

- Verify that dumpee continues running with correct registers after
  parasite injection:
  $ zdtm.py run --norst -t zdtm/static/s390x_regs_check
  $ zdtm.py run --norst --pre 2 -t zdtm/static/s390x_regs_check
  $ zdtm.py run --check-only -t zdtm/static/s390x_regs_check

Reviewed-by: Alice Frosi <alice@linux.vnet.ibm.com>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>