tests: refactor fw_cfg_test

tests: refactor fw_cfg_test

Currently, fw_cfg_test uses one QTestState for every test case.
This will add all command lines for every test case and
this is unnecessary. This patch split the test cases and for
every test case it uses his own QTestState. This patch does following
things:

1. Get rid of the global 'fw_cfg', this need add a uninit function

2. Convert every test case in a separate QTestState

After this patch, we can add fw_cfg test case freely and will not
have effect on other test cases.

Signed-off-by: Li Qiang <liq3ea@163.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190424140643.62457-2-liq3ea@163.com>
[PMD: Removed 'ret' local variable in main()]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>