dpif-netdev: Add parameters to configure PMD auto load balance.

dpif-netdev: Add parameters to configure PMD auto load balance.

Two important parts of how PMD auto load balance operates are how
loaded a core needs to be and how much improvement is estimated
before a PMD auto load balance can trigger.

Previously they were hardcoded to 95% loaded and 25% variance
improvement.

These default values may not be suitable for all use cases and
we may want to use a more (or less) aggressive rebalance, either
on the pmd load threshold or on the minimum variance improvement
threshold.

The defaults are not changed, but "pmd-auto-lb-load-threshold" and
"pmd-auto-lb-improvement-threshold" parameters are added to override
the defaults.

$ ovs-vsctl set open_vswitch . other_config:pmd-auto-lb-load-threshold="70"
$ ovs-vsctl set open_vswitch . other_config:pmd-auto-lb-improvement-threshold="20"

Signed-off-by: Christophe Fontaine <cfontain@redhat.com>
Co-Authored-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Ian Stokes <ian.stokes@intel.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>