win: Use correct format string for intptr_t
    
    Using %d as a format-specifier for intptr_t causes a warning with
    mingw64:
    virt-viewer-events.c: In function 'virt_viewer_events_add_handle':
    virt-viewer-events.c:103:5: warning: format '%d' expects argument of
    type 'int', but argument 5 has type 'intptr_t' [-Wformat=]
         g_debug("Converted fd %d to handle %d", fd, _get_osfhandle(fd));