qga-win32: Fix memory leak of device information set

The caller of SetupDiGetClassDevs must delete the returned device information
 set when it is no longer needed by calling SetupDiDestroyDeviceInfoList.

Signed-off-by: Li Ping <li.ping288@zte.com.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>