Fix the handling of VCPUs, specifically wrt the broken VCPU hotplugging, bug

Fix the handling of VCPUs, specifically wrt the broken VCPU hotplugging, bug
#280.  The cpu/<id>/availability paths had moved into /vm, but that is not
easily accessible by the hotplugging driver, so I have created a /vm entry
called vcpu_avail, so that the setting migrates along with the domain, and
moved the cpu/<id> ones back to /local/domain.

Don't try and destroy the domain twice if it fails within construct.  This
wasn't harming anything, but there's no need.

Signed-off-by: Ewan Mellor <ewan@xensource.com>