Allow all storage drivers to shrink volumes, like file-reflink

[rustybird]

Similar in purpose to #477, #478, #479 by @a-barinov in 2022, but with the shrinking check happening on the frontend side as suggested by @marmarek. The frontend already had such a check but only in the qvm-volume tool; QubesOS/qubes-core-admin-client#475 centralizes this check to qubesadmin.storage.Volume.resize() so that any other callers are protected as well. (Qube Manager's qube Settings dialog / qubes-vm-settings also has its own logic that avoids calling resize() if it would shrink.)

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.33%. Comparing base (befbb9b) to head (cee4105).
⚠️ Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
qubes/storage/lvm.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #835      +/-   ##
==========================================
- Coverage   70.46%   70.33%   -0.14%     
==========================================
  Files          61       61              
  Lines       14161    14137      -24     
==========================================
- Hits         9979     9943      -36     
- Misses       4182     4194      +12     

Flag	Coverage Δ
unittests	70.33% <83.33%> (-0.14%)	⬇️

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[rustybird]

@Rudd-O: Are you sure that OpenZFS's zfs set volsize=... - which the 'zfs' driver uses to resize volumes, right? - does not support shrinking? Here's someone in the OpenZFS forum (upvoted by two people) wishing that it didn't shrink without prompting for confirmation: openzfs/zfs#14820. And some people in the FreeNAS forum also claiming that it works: https://www.truenas.com/community/threads/changing-size-shrink-on-active-zvol.42142/#post-444130

Maybe it fails if snapshots of the zvol exist? If OpenZFS does support shrinking even then, I propose to add this commit to the PR: rustybird@shrink-volumes-zfs

[rustybird]

After setting up ZFS, I tested shrinking a 'zfs' driver volume and it worked fine. So I've added that commit to the PR.