vhost: remove payload size limitation

vhost: remove payload size limitation

FDs at the end of the VhostUserMessage structure limits the size
of the payload. Move them to an other englobing structure, before
the header & payload of a VhostUserMessage.
Also removes a reference to fds in the VHUMsg structure defined in
drivers/net/virtio/virtio_user/vhost_user.c

Signed-off-by: Christophe Fontaine <cfontain@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>