serial line access?

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I've got xen running on hardware that, when running 4.0 native (no xen
at all), says

com0 at isa0 port 0x3f8-0x3ff irq 4: ns16550a, working fifo
com1 at isa0 port 0x2f8-0x2ff irq 3: ns16550a, working fifo

This is when booted from sd0.  But when I plug in two IDE disks (so the
BIOS boots from one of them instead) and bring up a xen dom0, I get
only

com1 at isa0 port 0x2f8-0x2ff irq 3: ns16550a, working fifo

What happened to com0?  Can I get it back (in either dom0 or a domU)?
If so, how?

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