add vm_stop helper

add vm_stop helper

Since we use a post-stop hook to unmount all file systems at
container shutdown rather than a stop hook (because at this
point there are still multiple mount namespaces around), we
need to wait for the lxc-start/monitor process to exit to be
sure all the unmounting has succeeded, because it will put
the container into a STOPPED state before executing the
post-stop hook, making lxc-wait and lxc-stop signal success
too early when waiting for the container to stop.

Introduce a vm_stop() helper which calls lxc-stop and then
waits for the command socket to close. Note that lxc-stop
already has the "hard-stop-after-timeout" mechanic built in,
so the 'forceStop' code path of the vm_stop api call removed
here was not actually necessary.
Technically we could pass --nokill for the behavior assumed
there, but for now this patch should not be causing any
actual behavior changes.