avocado_qemu: Fix KNOWN_DISTROS map into the LinuxDistro class

avocado_qemu: Fix KNOWN_DISTROS map into the LinuxDistro class

As the KNOWN_DISTROS grows, more loosely methods will be created in
the avocado_qemu/__init__.py file.

Let's refactor the code so that KNOWN_DISTROS and related methods are
packaged in a class

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20210706131729.30749-2-eric.auger@redhat.com>
[CR: moved aarch64 definition from patch 2 to 1]
[CR: protect get() when arch is not defined]
[CR: split long lines]
Acked-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>