vzmigrate: don't hardcode /vz/lock, use LOCKDIR

As pointed out in bug #2976, vzmigrate use hardcoded /vz/lock
instead of respecting LOCKDIR in vz.conf. Fix it.

This is a fix to commit 7ce7733.

https://bugzilla.openvz.org/2976

Reported-by: Denis Ognewsky <emp@zra.ru>
Reported-by: Pete Foster <xmobuk@gmail.com>
Reported-by: Alexander Tsoy <alexander@tsoy.me>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>