libvhost-user: add glib source helper

This file implements a bridge from the vu_init API of libvhost-user to
GSource, so that libvhost-user can be used inside a GLib main loop.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
