qemu-char: Use qemu_open() to avoid leaking fds to children

Fixed silently in commit aad04cd0, but that just got reverted.
Re-apply the fixes, plus one missed instance: parport on Linux.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
