qga: Fix mingw compilation warnings on enum conversion

The win2qemu[] is supposed to be the conversion table to convert between
STORAGE_BUS_TYPE in Windows SDK and GuestDiskBusType in qga.

But it was incorrectly written that it forces to set a GuestDiskBusType
value to STORAGE_BUS_TYPE, which generates an enum conversion warning in clang.

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Cao Jiaxi <driver1998@foxmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20190503003650.10137-1-driver1998@foxmail.com
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>