openvz: check the exitstatus of vzlist

openvz: check the exitstatus of vzlist

I noticed this while auditing all calls to virCommandRun that request
an exit status from virCommandRun. Two functions in the openvz driver

  openvzDomainGetBarrierLimit
  openvzDomainSetBarrierLimit

request an exit status from virCommandRun (thus assuring that
virCommandRun won't log any errors just due to a non-0 exit status),
but then fail to examine that exit status. This could result in the
functions believing that the call to "vzlist" was successful, even
though it may have encountered an error.