nx-match: Serialize standard xregs instead of Nicira registers, in OF1.5.

Commit 79fe0f4611b60 (meta-flow: Add 64-bit registers.) added support for
the OpenFlow 1.5 (draft) standardized registers, but neglected to cause
them to be serialized when Open vSwitch composes flow matches.  This meant
that they were always sent to a controller as pairs of Nicira extension
registers.  This commit fixes the problem.

Found by inspection.

ONF-JIRA: EXT-244
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>