virttest.aexpect: Support ignore_status parameter in cmd

 virttest.aexpect: Support ignore_status parameter in cmd().
    
    When run some clean up command in guest. We may do not want to fail the case
    for any exception in the command.
    So add ignore_status parameter in cmd() to ignore exception.

    qemu/tests/stop_continue call session.call() with ignore_status=True.
    We do not want to fail the case for exception in clean up command.