tests/avocado: Allow overwrite smp and memory size command line options

tests/avocado: Allow overwrite smp and memory size command line options

Removes the hard-coded values in setUp(). Class inheriting from
avocado_qemu.LinuxTest can overwrite the default smp and memory instead.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/453
Signed-off-by: Ahmed Abouzied <email@aabouzied.com>
Message-Id: <20210802222257.50946-1-email@aabouzied.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>