Background:- I have tested a Gentoo Virtual machine (DOMU) to access the serial port of Physical machine (opensuse 11.1) (DOM0) using the following configuration.

Step1: Following modules should be loaded on the DOM0
8250
8250_pnp
serial_core

check is using "lsmod" command.

Step 2:- Finding the IRQ and ioport ranges.

Run the following command on the DOM0 to find the irq for the serial port.

# dmesg |grep tty
serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A

and run the following command to find the ioport


# cat /proc/ioports |grep serial
02f8-02ff : serial
03f8-03ff : serial

now add the following lines in the DOMU config file ( i mean configuration file of virtual machine)


domU config:
irq = [ 4 ]
ioports = [ "02f8-02ff" ]

check with minicom

if you are unsuccessful then try the other value.