qemu: add reporting of vCPU wait time
    
    The VIR_DOMAIN_STATS_VCPU flag to virDomainListGetStats
    enables reporting of stats about vCPUs. Currently we
    only report the cumulative CPU running time and the
    execution state.
    
    This adds reporting of the wait time - time the vCPU
    wants to run, but the host scheduler has something else
    running ahead of it.
    
    The data is reported per-vCPU eg
    
    $ virsh domstats --vcpu demo
     Domain: 'demo'
       vcpu.current=4
       vcpu.maximum=4
       vcpu.0.state=1
       vcpu.0.time=1420000000
       vcpu.0.wait=18403928
       vcpu.1.state=1
       vcpu.1.time=130000000
       vcpu.1.wait=10612111
       vcpu.2.state=1
       vcpu.2.time=110000000
       vcpu.2.wait=12759501
       vcpu.3.state=1
       vcpu.3.time=90000000
       vcpu.3.wait=21825087
    
    In implementing this I notice our reporting of CPU execute
    time has very poor granularity, since we are getting it
    from /proc/$PID/stat. As a future enhancement we should
    prefer to get CPU execute time from /proc/$PID/schedstat
    or /proc/$PID/sched (if either exist on the running kernel)
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>