qemu: wire up standalone VNC in driver APIs

qemu: wire up standalone VNC in driver APIs

Route the OpenGraphics, OpenGraphicsFD, and GraphicsReload driver
APIs through the standalone qemu-vnc D-Bus interface when active.
Client connections are passed via D-Bus fd-passing (AddClient),
certificate reloading uses the ReloadCertificates method, and
password changes use SetPassword.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>