testing and gdbstub updates: - add a check-rust test to docker builds - re-factor the qtest logic to be cleaner - fix tests to not clock_step when no timers enabled - roll-up log prefix into qtest_send - cleaner error reporting when qtest_clock_set fails - revert old deadlock fix now tests are updated - only run full set of migration tests under HW acceleration - support late attachment to user-mode gdbstubs