tests/vm: Added a new script for ubuntu.aarch64.

tests/vm: Added a new script for ubuntu.aarch64.

ubuntu.aarch64 provides a script to create an Ubuntu 18.04 VM.
Another new file is also added aarch64vm.py, which is a module with
common methods used by aarch64 VMs, such as how to create the
flash images.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Peter Puhov <peter.puhov@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200601211421.1277-6-robert.foley@linaro.org>
Message-Id: <20200701135652.1366-10-alex.bennee@linaro.org>