tests/qtest/boot-serial-test: Initialize PL011 Control register

tests/qtest/boot-serial-test: Initialize PL011 Control register

The tests using the PL011 UART of the virt and raspi machines
weren't properly enabling the UART and its transmitter previous
to sending characters. Follow the PL011 manual initialization
recommendation by setting the proper bits of the control register.

Update the ASM code prefixing:

  *UART_CTRL = UART_ENABLE | TX_ENABLE;

to:

  while (true) {
      *UART_DATA = 'T';
  }

Note, since commit 51b61dd4d56 ("hw/char/pl011: Warn when using
disabled transmitter") incomplete PL011 initialization can be
logged using the '-d guest_errors' command line option.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>