qemu_passt: Let passt write the PID file

qemu_passt: Let passt write the PID file

The way we start passt currently is: we use
virCommandSetPidFile() to use our virCommand machinery to acquire
the PID file and leak opened FD into passt. Then, we use
virPidFile*() APIs to read the PID file (which is needed when
placing it into CGroups or killing it). But this does not fly
really because passt daemonizes itself. Thus the process we
started dies soon and thus the PID file is closed and unlocked.

We could work around this by passing '--foreground' argument, but
that weakens passt as it can't create new PID namespace (because
it doesn't fork()).

The solution is to let passt write the PID file, but since it
does not lock the file and closes it as soon as it is written, we
have to switch to those virPidFile APIs which don't expect PID
file to be locked.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>