Merge remote-tracking branch 'mdroth/qga-pull-4-2-13' into staging

# By Laszlo Ersek (2) and others
# Via Michael Roth
* mdroth/qga-pull-4-2-13:
  qemu-ga: ga_get_fd_handle(): abort if fd_counter overflows
  qga schema: document generic QERR_UNSUPPORTED
  qga schema: mark optional GuestLogicalProcessor.can-offline with #optional
  qga: add windows implementation for guest-set-time
  qga: add windows implementation for guest-get-time