qemu: remove dbus-vmstate code

qemu: remove dbus-vmstate code

This code was based on a per-helper instance and peer-to-peer
connections. The code that landed in qemu master for v5.0 is relying
on a single instance and DBus bus.

Instead of trying to adapt the existing dbus-vmstate code, let's
remove it and resubmit. That should make reviewing easier.

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