qemu-file: Use qemu_file_is_writable() on stdio_fclose()

Use the existing function which checks if writev_buffer() or
put_buffer() are set, instead of duplicating it.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
