qga: Fix mingw compilation warnings on enum conversion

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 <[email protected]>
Signed-off-by: Cao Jiaxi <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-id: 20190503003650[email protected]
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>