fix power_readings tag ambuguity in nvidia-smi xml

[J-HowHuang]

This is the output snippet from nvidia-smi -x -q with driver version 525.125.06

...
<power_readings>
    <power_state>P5</power_state>
    <power_management>Supported</power_management>
    <power_draw>37.29 W</power_draw>
    <power_limit>420.00 W</power_limit>
    <default_power_limit>420.00 W</default_power_limit>
    <enforced_power_limit>420.00 W</enforced_power_limit>
    <min_power_limit>100.00 W</min_power_limit>
    <max_power_limit>450.00 W</max_power_limit>
</power_readings>
...

Not sure starting from which version of driver that they slightly changed the xml document schema to this (this was generated with 535.104.05)

...
<gpu_power_readings>
    <power_state>P8</power_state>
    <power_draw>17.89 W</power_draw>
    <current_power_limit>220.00 W</current_power_limit>
    <requested_power_limit>220.00 W</requested_power_limit>
    <default_power_limit>220.00 W</default_power_limit>
    <min_power_limit>100.00 W</min_power_limit>
    <max_power_limit>240.00 W</max_power_limit>
</gpu_power_readings>
<module_power_readings>
    <power_state>P8</power_state>
    <power_draw>N/A</power_draw>
    <current_power_limit>N/A</current_power_limit>
    <requested_power_limit>N/A</requested_power_limit>
    <default_power_limit>N/A</default_power_limit>
    <min_power_limit>N/A</min_power_limit>
    <max_power_limit>N/A</max_power_limit>
</module_power_readings>
...

The xml extraction part in gpu/nvidia which tries to get power draw from nvidia-smi needs to be changed from locating the "power_readings" tag to the "gpu_power_readings" tag to support newer version of nvidia driver.