Code review comment for ~newell-jensen/maas:lp1789521

I tested this branch and found that it doesn't work when composing VMs from the Web UI. I get the following error:

    Database connections in this thread (MainThread) are disabled.

The code that was added inside get_requested_machine() will need to be placed in a separate function and called within a deferToDatabase function when isInIOThread() turns out to be true.