libxl: remove QMP warning when no pty style serial device is configured

libxl: remove QMP warning when no pty style serial device is configured

If the serial device is not "pty" then there is nothing to do but this is not
an error. Removes a spurious warning if e.g. serial="file:/tmp/serial.log" is used:

libxl: error: libxl_qmp.c:137:register_serials_chardev_callback: Failed to store serial port information in xenstore: No such file or directory

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>