According to net/Makefile.objs we only link in the vhost-user code
if CONFIG_POSIX has been set. So the help screen should also only
show this information if CONFIG_POSIX has been defined.
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Thomas Huth <thuth@redhat.com> Signed-off-by: Jason Wang <jasowang@redhat.com>