Code review comment for lp:~allenap/maas/xml-as-bytes--bug-1570609

I tested with AMT version 8.1.57 as follows:

In [1]: import provisioningserver.drivers.power.amt as amt
In [2]: driver = amt.AMTPowerDriver()
In [3]: driver.power_off(None, {'ip_address': '172.16.100.251', 'power_pass': 'xxx'})
In [4]: driver.power_on(None, {'ip_address': '172.16.100.251', 'power_pass': 'xxx', 'boot_mode': 'local'})
In [9]: driver.power_query(None, {'ip_address': '172.16.100.251', 'power_pass': 'xxx', 'boot_mode': 'local'})
Out[9]: 'on'

I visually verified that it worked.