borealis: Improve free disk space checks for LVM based layouts

Use spaced as the source of truth for free disk space checks.
Additionally, use "/home/chronos/user" as the actual path to check
free space against; since some VM will be using the user cryptohome
as a source of storage, the maximum disk space they can use will
also be limited by the space taken up by the user filesystem.

Bug: b:251028366
Test: unittests, tast run borealis.*
Change-Id: I4d0fc94aac0f46f787e6898de378acec3d23656f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3958195
Commit-Queue: Daniel Ng <danielng@google.com>
Auto-Submit: Sarthak Kukreti <sarthakkukreti@chromium.org>
Reviewed-by: Daniel Ng <danielng@google.com>
Cr-Commit-Position: refs/heads/main@{#1060301}