API: Fix shared memory only action handlers.

Some API action handlers called vl_msg_ai_send_shmem()
directly. That breaks Unix domain socket API transport.

A couple (bond / vhost) also tried to send a sw_interface_event
directly, but did not send the message to all that had
registred interest. That scheme never worked correctly.
Refactored and improved the interface event code.

Change-Id: Idb90edfd8703c6ae593b36b4eeb4d3ed7da5c808
Signed-off-by: Ole Troan <ot@cisco.com>