vhost: validate FDs attached to messages

vhost: validate FDs attached to messages

Some message handlers do not expect any file descriptor attached as
ancillary data.
Provide a common way to enforce this by adding a accepts_fd boolean in
the message handler structure. When a message handler sets accepts_fd to
true, it is responsible for calling validate_msg_fds with a right
expected file descriptor count.
This will avoid leaking some file descriptor by mistake when adding
support for new vhost user message types.

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