borealis: revert and update request space changes

When modifying request space to regenerate the buffer, we also
changed how the response was calculated. We changed from delta
in disk size to delta in available space. This is problematic
because the VM may consume space during the resize request.
Reverting this change so that we use disk size again, but we're
also updating this to exclude the space needed to regenerate the
buffer from our calculations.
Previous CL: https://crrev.com/c/3423405

Bug: b/226690957
Change-Id: I40db386cbfbb0f0f30e5bc5532084d06663732c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3551482
Reviewed-by: Chloe Pelling <cpelling@google.com>
Reviewed-by: Nic Hollingum <hollingum@google.com>
Commit-Queue: Daniel Ng <danielng@google.com>
Cr-Commit-Position: refs/heads/main@{#987796}