compel: Add compel_run_at helper

Simply run tracee from specfied IP assuming
it's arelady have trapping instruction in
stream.

It's unsafe low-level function use with caution.

travis-ci: success for compel: A fix and new helper
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>