ofp-actions: Better support OXM in Copy-Field action.

ofp-actions: Better support OXM in Copy-Field action.

The OpenFlow 1.5 (draft) Copy-Field action has two OXM headers, one after
the other.  Until now, Open vSwitch has implemented these as a pair of
ovs_be32 members, which meant that only 32-bit OXM could be supported.  This
commit changes the implementation to use nx_pull_header(), which means that
in the future when that function supports 64-bit experimenter OXMs,
Copy-Field will automatically get that support too.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>