Merge tag 'migration-20240331-pull-request' of https://gitlab.com/peterx/qemu into staging

Migration pull for 9.0-rc2

- Avihai's two fixes on error paths

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* tag 'migration-20240331-pull-request' of https://gitlab.com/peterx/qemu:
  migration/postcopy: Ensure postcopy_start() sets errp if it fails
  migration: Set migration error in migration_completion()

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>