fix #2086: change process checking mechanism in vmstatus

fix #2086: change process checking mechanism in vmstatus

vmstatus checked if the container was running by looking at the pid,
which was not an indicator of the process being completely stopped, as
the command socket in /proc/net/unix stays a little while after the
process is dead according to lxc-info.

this resulted in destroy_vm and similar functions which use
/proc/net/unix command socket based checking mechanism to fail when
executed too fast after the vm_status reported the process as stopped.

this changes vm_status to use the same kind of command socket based
mechanism in order to avoid reporting the container as being stopped too
early.

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>