bench: the frontend path is in the frontend (!)

This was causing unnecessary transaction conflicts and is just wrong.

Signed-off-by: David Scott <dave.scott@citrix.com>
bench: each VBD has a distinct devid (previously they were all 0)

Signed-off-by: David Scott <dave.scott@citrix.com>
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 <dave.scott@citrix.com>