Improve interaction between volume attachment controller and migrations

[ejweber]

Which issue(s) this PR fixes:

longhorn/longhorn#8735

What this PR does / why we need it:

See context at https://github.com/longhorn/longhorn/issues/8735#issue-2347139063 and implementation plan at https://github.com/longhorn/longhorn/issues/8735#issuecomment-2177309258.

~This is a draft for now as I work on testing it in all situations.~

[ejweber]

The behaviors in this PR now match the test cases from https://github.com/longhorn/longhorn-tests/pull/1948. It is ready for review.

[PhanLe1010]

I will review this ~~today~~ on Monday