Avoid reporting an error if veth device is already deleted

Avoid reporting an error if veth device is already deleted

The kernel automatically destroys veth devices when cleaning
up the container network namespace. During normal shutdown, it
is thus likely that the attempt to run 'ip link del vethN'
will fail. If it fails, check if the device exists, and avoid
reporting an error if it has gone. This switches to use the
virCommand APIs instead of virRun too.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>