Power-off, instead of halt, when loader can't run command

Currently, when we try to run an invalid shared object (e.g, run.py -e aaa)
loader.cc calls abort(). This patch changes it to use osv::poweroff().

This is useful, for example, to measure how much time our boot/poweroff
cycle takes, without running any payload, by doing

	time scripts/run.py -e aaa

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>