Limit PVC block mode logic to non-Windows platform

[blackpiglet]

Release v1.12.rc.1 failed with the Windows binary build. The error message is:

# github.com/vmware-tanzu/velero/pkg/uploader/kopia
pkg/uploader/kopia/block_backup.go:41:31: undefined: syscall.Stat_t
pkg/uploader/kopia/block_restore.go:94:31: undefined: syscall.Stat_t

The reason is that PVC block mode backup and restore introduced some OS-specific system calls.
Those calls are not available for Windows, so add both non-Windows version and Windows version code and return an error for block mode on the Windows platform.

Thank you for contributing to Velero!

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Does your change fix a particular issue?

Fixes #(issue)

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• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

Merging #6986 (3034cdb) into release-1.12 (353ff55) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           release-1.12    #6986      +/-   ##
================================================
- Coverage         60.47%   60.44%   -0.04%     
================================================
  Files               247      247              
  Lines             26398    26398              
================================================
- Hits              15965    15957       -8     
- Misses             9312     9319       +7     
- Partials           1121     1122       +1     

Files	Coverage Δ
pkg/uploader/kopia/block_backup.go	30.00% <ø> (ø)
pkg/uploader/kopia/block_restore.go	21.27% <ø> (ø)

... and 1 file with indirect coverage changes

[blackpiglet]

@dzaninovic
Hi David, please also help to take a look at this PR.

[dzaninovic]

@blackpiglet, fix looks good to me.

[anshulahuja98]

@Lyndon-Li should we add this to public docs?

[blackpiglet]

@anshulahuja98
What information would you like to add?
IMO, this change is not visible to users, so there seems no need to mention it in the document.

[Lyndon-Li]

I think it is about adding a limitation statement for Windows, is that what you mean? @anshulahuja98
I see the doc indeed needs to change as it is still saying "block mode is not supported". Let me correct it.

[anshulahuja98]

I think it is about adding a limitation statement for Windows, is that what you mean? @anshulahuja98
I see the doc indeed needs to change as it is still saying "block mode is not supported". Let me correct it.

Yes it was for windows limitation.

[blackpiglet]

@Lyndon-Li @anshulahuja98
I see. I will create a PR to record that in the documents.

[anshulahuja98]

Even the changelog / pr title of current pr is misleading

[blackpiglet]

@anshulahuja98 @Lyndon-Li
PR is created to record the platform limitation: #7013.