tests/avocado: raspi2_initrd: Wait for guest shutdown message before stopping

tests/avocado: raspi2_initrd: Wait for guest shutdown message before stopping

The avocado test
 tests/avocado/boot_linux_console.py:BootLinuxConsole.test_arm_raspi2_initrd
finishes with

    exec_command(self, 'halt')
    # Wait for VM to shut down gracefully
    self.vm.wait()

In theory this should be fine. In practice it runs into two bugs:

 * when the test calls self.vm.wait() Avocado closes the socket
   connection to the guest serial console immediately, so the
   avocado logs don't have the last part of the guest output:
   https://gitlab.com/qemu-project/qemu/-/issues/1265
 * when the socket is closed, a bug in the QEMU socket chardev
   means that it loses any data that the guest UART has not
   yet consumed. This means that the guest doesn't always read
   the full 'halt' command string, so the test intermittently
   fails with a timeout:
   https://gitlab.com/qemu-project/qemu/-/issues/1264

Work around both of these by waiting for the guest to print the
string that means it has completed the shutdown process.  This fixes
a very long standing intermittent failure in this test.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/636
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20221020102012.3015662-1-peter.maydell@linaro.org>
Message-Id: <20221027183637.2772968-20-alex.bennee@linaro.org>