Add a disk to a guest

First you may want to clone the image you are working with.

Next create the file(s) to be used as the additional disk(s).

/etc/xen/

name = "guest"
uuid = "5bf34997-75d1-7c48-4fd5-8bb28290a7bb"
maxmem = 512
memory = 512
vcpus = 1
bootloader = "/usr/bin/pygrub"
on_poweroff = "destroy"
on_reboot = "restart"
on_crash = "restart"
vfb = [ "type=vnc,vncunused=1,keymap=en-us" ]
disk = [ "tap:aio:/var/lib/xen/images/guest.img,xvda,w", "tap:aio:/var/lib/xen/images/disk1.img,xvdb,w" ]
vif = [ "mac=00:12:3e:70:f5:5b,bridge=xenbr0" ]