gtools: Fix a race in the cpustats collectd plugin

If the VM has just been started, the cpustats collectd plugin might try
to read /proc/<pid>/fd/ for VCPUs fds, before qemu has initialized /
created the VCPUs. This led to a ZeroDivision error in the collectd
plugin. Make sure to skip that particular VM until the VCPUs have been
created.