tests/qtest/migration: Add a cancel test

tests/qtest/migration: Add a cancel test

The qmp_migrate_cancel() command is poorly tested and code inspection
reveals that there might be concurrency issues with its usage. Add a
test that runs a migration and calls qmp_migrate_cancel() at specific
moments.

In order to make the test more deterministic, instead of calling
qmp_migrate_cancel() at random moments during migration, do it after
the migration status change events are seen.

The expected result is that qmp_migrate_cancel() on the source ends
migration on the source with the "cancelled" state and ends migration
on the destination with the "failed" state. The only exception is that
a failed migration should continue in the failed state.

Cancelling is not allowed during postcopy (no test is added for this
because it's a trivial check in the code).

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-ID: <20250213175927.19642-9-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>