fix freespaceGreater check sent to agent

This check was sent to the agent Bytes amount instead of MiB.

I've also refactored size calculation to use getUnitSize() method and also to disallow KiB or B
because it's based on MiB and allowing those 2 choices means we'll send a float value to compare to
the agent.

fixes #2058
Merge remote-tracking branch 'fusinv/master'