Cannot change power settings for machine

In the UI I tried to change a node from virsh to manual power control.
Apparently this worked, but later I noticed that it had not. Checking
the logs shows what happened:

Error on request (89) machine.update: {"mac_addresses": ["This field is required."], "architecture": ["'i386/generic' is not a valid ar
chitecture. It should be one of: 'amd64/generic'."]}
Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 862, in run
    self._target(*self._args, **self._kwargs)
  File ".../src/provisioningserver/utils/twisted.py", line 842, in worker
    return target()
  File "/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
    task()
  File "/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
    task()
--- <exception caught here> ---
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File ".../src/maasserver/utils/orm.py", line 516, in call_within_transaction
    return func_outside_txn(*args, **kwargs)
  File ".../src/maasserver/utils/orm.py", line 343, in retrier
    return func(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File ".../src/maasserver/websockets/handlers/machine.py", line 262, in update
    data = super(NodeHandler, self).update(params)
  File ".../src/maasserver/websockets/base.py", line 399, in update
    raise HandlerValidationError(form.errors)
maasserver.websockets.base.HandlerValidationError: {'mac_addresses': ['This field is required.'], 'architecture': ["'i386/generic' is not a valid architecture. It should be one of: 'amd64/generic'."]}

There are two bugs here:

- The UI was misleading; the error was not shown to me. This is a known
  problem (bug 1556963) that affects all of the UI.

- Editing power parameters in the UI is a separate interaction to
  editing MACs or setting the architecture, so it's incorrect to
  validate those fields when editing power parameters.