Move check of TLV length for management COMMAND messages

Currently, it is assumed that the management TLV data of management COMMAND
messages is always empty. This is not true for the INITIALIZE command and
also for a custom command we'll be introducing.

Move the check to msg_post_recv and let it check only the TLVs defined by
the standard.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Move common code into port_prepare_and_send

The task of preparing the message for transmission and sending it appears
at many places. Unify them into a new function.

Signed-off-by: Jiri Benc <jbenc@redhat.com>