Use xm block-list <domid> to find the cdrom be-path for the domain, for example:
localhost:~# xm block-list 3
Vdev BE handle state evt-ch ring-ref BE-path
768 0 0 1 -1 -1 /local/domain/0/backend/vbd/3/768
5632 0 0 1 -1 -1 /local/domain/0/backend/vbd/3/5632
Having identified the cdrom device (5632) you can check what iso image it is connected to:
localhost:~# xenstore-read /local/domain/0/backend/vbd/3/5632/params
/root/win/win.ISO
To connect a new iso image:
xenstore-write local/domain/0/backend/vbd/3/5632/params /mnt/new.iso
And you can now see that it is connected:
xenstore-read /local/domain/0/backend/vbd/3/5632/params /mnt/new.iso
This method works with both emulated devices and with gplpv drivers.