Update VM's VCPU handling to match the documentation wrt VCPUs_max and

Update VM's VCPU handling to match the documentation wrt VCPUs_max and
VCPUs_at_startup.  Add VM.set_VCPUs_number_live, VM.add_to_VCPUs_params_live,
VM_metrics.VCPUs_{CPU,flags,params}.

Use these new calls to implement xm vcpu-list and xm sched-credit through
the Xen-API.  Mark a number of other calls as unimplemented through that API.

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>
Signed-off-by: Ewan Mellor <ewan@xensource.com>