char-win: do not override chr_free

For some unclear reason to me, char-file does not have chr_free on
win32. Since we want to switch to instance finalizer instead of class
chr_free, we should be able to run the base WinChardev class finalizer
in any case. Use a boolean to skip free to ease the transition to
instance finalizer.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
