vhost: fix external message handlers

vhost: fix external message handlers

Following a rework, external message handlers were receiving a pointer
to a vhost_user message (as stated in the API), but lost the ability to
interact with fds attached to the message.
Restore the original layout and put a build check and reminders.

Bugzilla ID: 953
Fixes: 5e0099dc709e ("vhost: remove payload size limitation")
Reported-by: Fan Zhang <roy.fan.zhang@intel.com>
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Jakub Poczatek <jakub.poczatek@intel.com>
Acked-by: Jakub Poczatek <jakub.poczatek@intel.com>
Reviewed-by: Christophe Fontaine <cfontain@redhat.com>