libxl: Advertise support for VIR_MIGRATE_CHANGE_PROTECTION
The libxl driver has basic support for VIR_MIGRATE_CHANGE_PROTECTION
by starting and stopping modify jobs in the begin/confirm and prepare/finish
phases of migration, but it doesn't advertise that support. This can result
in unterminated jobs because the migration logic skips phases of migration
when the VIR_MIGRATE_CHANGE_PROTECTION feature is absent. Ensure jobs are
terminated properly by advertising support for VIR_MIGRATE_CHANGE_PROTECTION.