Update used btrfs file system commands, new minimum is btrfs-progs 4.5

Merged Mike Fleetwood requested to merge mfleetwo/gparted:btrfs-cmd-updates into master

Update btrfs commands to:

  1. avoid rounding of figures when reading FS usage;
  2. allow reading FS usage, UUID and labels from file system images allowing relevant unit tests to be enabled.
Edited by Mike Fleetwood

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