Howto use the serial ports (/dev/ttySX) from a xen domU

I've recently migrated a zabbix server from a standalone machine to a xen vm.
Attached to serial port of the old server there was a GSM modem used by smstools to send via SMS some of zabbix alarms.
So I wanted to use the same modem with the new domU zabbix server: this is my little howto to get a xen domU access serial ports.

1. First of all we have to avoid the dom0 to access the serial ports of the machine:
   with debian create a new file (eg.serial-blacklist) in /etc/modprobe.d/ specifying the blacklisting for 3 kernel modules related to serial ports

   blacklist 8250
   blacklist 8250_pnp
   blacklist serial_core
   

2. Configure the vm: in the configuration file for the domU specify the following lines:

   irq = [4]
   ioports = ['3f8-3ff']
   

3. On the domU filesystem verify the availability of the kernel modules in /lib/modules/`uname -r`/kernel/drivers/serial:

   8250.ko  8250_pci.ko  8250_pnp.ko  jsm  serial_core.ko  serial_cs.ko
   

4. In order to have the correct devices in /dev directory install udev:

   apt-get install udev