openvz: convert popen to virCommand

openvz: convert popen to virCommand

popen must be matched with pclose (not fclose), or it will leak
resources.  Furthermore, it is a lousy interface when it comes to
signal handling.  We're much better off using our decent command
wrapper.  Note that virCommand guarantees that VIR_FREE(outbuf) is
both required and safe to call, whether virCommandRun succeeded or
failed.

* src/openvz/openvz_conf.c (openvzLoadDomains, openvzGetVEID):
Replace popen with virCommand usage.