mssqlvdi-fd: add support for filestream backups

[sebsura]

Thank you for contributing to the Bareos Project!

The plugin was written during a time where the VDC_Flush command was only emitted at the end of a backup.
This caused our logic to conflate the two: whenever VDC_Flush was requested, we told the core (implicitly by returning count=0 from read) that the single file we backup is done.

As mentioned in this article this is (and was) always a bad assumption.

Fixing this without changing the complete logic of the plugin was hard, but thanks to the introduction of the VDC_Complete command it was possible.

The way it works is like so: When starting a backup, we request from the vdi device that it should notify us when the backup is complete by setting VDF_RequestComplete in the configuration.
If this succeeds (completion_support = true) then during IO operations we ignore VDC_Flush and continue on until we hit VDC_Complete at which point we actually tell the core that we are done.

This also fixes a small mistake from #2120.

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[sebsura]

We should also have a test that actually back ups/ restores a filestream object.

[bruno-at-bareos]

In this last review I've added or confirmed some request changes asked by @sebsura
A complete manual test on a separated host with database with other name has been successfully backup-ed and restored. Especially checked the recreation of the Attachment folder.

Plus we need to address the last bits of complain from pr-tool

 ✗  Commit checks failed:
        221771925 [fixup] tests: add debug logging: headline starts with '[fixup]'
        fcad22f9c [fixup] remove `set -x`: headline starts with '[fixup]'
 ✗  bareos-check-sources --since=13c0881293fcb435b9aeb1dcce42f124ba3bfd65 reported:
        Plugin 'fix copyright year' would modify 'core/src/plugins/filed/CMakeLists.txt'

[bruno-at-bareos]

We're almost done. I requested a few changes please check them.

[bruno-at-bareos]

final review, with approval.