There's no performance data return from zvm management tool if the
virtual machine in shutdown stat.
In current implementation, InstanceNotFoundException would be
raised in this case.
With this change integrated, zvm inspector will check instance's
power_state at first, then try to get the data.
Also did some change to ignore invalid data format exception that
in case of all virtual machines on the compute node are in shutdow
state.
Change-Id: I02b69adc6e818a69d1e6887a3344f6bf0b344aa7
dddf5f281b