bench: start domUs from domid 1; domid 0 is used for backends
bench: start domUs from domid 1; domid 0 is used for backends

Otherwise conflicts would be created by creating domid 0 at the same
time as trying to use it as a backend (!)

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

djs55/ocaml-xenstore-server,mirage/ocaml-xenstore-server,mirage/ocaml-xenstore-server