tests/avocado/boot_linux.py: Bump aarch64 virt test timeout to 720s

tests/avocado/boot_linux.py: Bump aarch64 virt test timeout to 720s

The two tests
tests/avocado/boot_linux.py:BootLinuxAarch64.test_virt_tcg_gicv2
tests/avocado/boot_linux.py:BootLinuxAarch64.test_virt_tcg_gicv3

take quite a long time to run, and the current timeout of 240s
is not enough for the tests to complete on slow machines:
we've seen these tests time out in the gitlab CI in the
'avocado-system-alpine' CI job, for instance. The timeout
is also insufficient for running the test with a debug build
of QEMU: on my machine the tests take over 10 minutes to run
in that config.

Push the timeout up to 720s so that the test definitely has
enough time to complete.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>