gdb ./vmlinux
set remotebaud 115200
target remote /dev/pts/? # from virsh ttyconsole <domain>

also add kgdboc=ttyS0,15200 (where that's what I'm using for the
serial console) to the boot commandline.
(But that didn't actually work for me for some bizarre reason I haven't
found yet, so I did an 

	echo ttyS0,15200 >/sys/module/kgdboc/parameters/kgdboc

on boot instead.)

You can test that it's working with echo c >/proc/sysrq-trigger (which
triggers a null deference).