openflow.nicira: header_only pack for nxm_entries

It use useful in some cases to be able to pack only the
header of an nxm_entry.  This is now supported.

This also fixes a bug that kept nx_reg_load actions
from working.

Thanks to Saul St. John for pointing out the issue.
openflow.nicira: Fix _eq in nx_reg_load

This was doing a bad job for checking equality of
nx_reg_load actions.