Reportstate volume size floc 976

Fixes FLOC-976
@itamarst I'm not 100% certain what I'm doing here is correct; can I take a reported filesystem size of `VolumeSize(maximum_size=0)` to be the same as `VolumeSize(maximum_size=None)`? Max size of zero could only other possibly mean.....what? Read only? Functional tests fail if I don't include this code, comments appreciated.