ofproto: Allow ofproto_delete_flow() to delete hidden rules.

Commit 97f63e57a8 (ofproto: Remove soon-to-be-invalid optimizations.)
changed ofproto_delete_flow() to use the general-purpose flow_mod
implementation.  However, the general-purpose flow_mod never matches hidden
flows, which are exactly the flows that ofproto_delete_flow()'s callers
want to delete.

This commit fixes the problem by allowing flow_mods that specify a priority
that can only be for a hidden flow to delete a hidden flow.  (This doesn't
allow OpenFlow clients to meddle with hidden flows because OpenFlow uses
only a 16-bit field to specify priority.)

I verified that, without this commit, if I change from one controller to
another with "ovs-vsctl set-controller", then the in-band rules for the
old controller remain.  With this commit, the old rules are removed.

Reported-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Bug #19821.
Reported-by: Luca Giraudo <lgiraudo@vmware.com>
Bug #19888.
Reported-by: Ying Chen <yingchen@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>