acpi: move aml builder code for serial device

The code uses the isa_serial_io array to figure what the device uid is.
Side effect is that acpi antries are not limited to port 1+2 any more,
we'll also get entries for ports 3+4.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200515150421.25479-6-kraxel@redhat.com>