Parallelise vif hotplug with domain build

We add the xenstore tree before we kick off the domain builder
so both happen in parallel. We still synchronise with the hotplug
at the end but we can usually avoid xenstore watches since the
hotplug has usually completed already.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Don't add a console to save time

This isn't strictly needed, although it makes guest debugging
difficult.

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