ofproto-dpif: Handle dest mirrors in compose_output_action().

ofproto-dpif: Handle dest mirrors in compose_output_action().

Before this patch, the mirroring code would retroactively insert
actions for destination mirrors after actions were translated.
This relied on converting datapath output actions into ofports
which doesn't work for tunnels and patch ports.  This patch
refactors the code to handle destination mirrors at output.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>