Add a new test in tests/avocado to check LoongArch virt machine start.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Message-Id: <20230513012744.1885728-1-gaosong@loongson.cn>