Make sure the default password, raspberry, is actually the default password for your pi operating system.
I just realized, for example, that the pi-top operating system has a default password of pi-top, which is a super annoying thing for them to do, but it is what it is.
I'm going through the same process right now and havn't compeleted it yet.
For the xen-rpi-builder I think they missed the latest XEN and Kernel and I think the patches may no longer be relevant so I upgraded the contained versions to the latest 4.14 and kernel 5.10.y. Not sure yet if this is good or bad but at least I could get it to build.
I think in order ...
For anyone who needs it in the future to find the needed information to mount the partition on the card you can use the already mentioned blkid or fdisk. This if the card does not auto mount for you.
root@zeus-H370M-DS3H:~# fdisk -l
Disk /dev/sde: 29.81 GiB, 32010928128 bytes, 62521344 sectors
Disk model: Storage Device
Units: sectors of 1 * 512 = ...
Just found out about the "lsblk" command.
It showed me that the SD card was automatically mounted to /media/pi/boot despite it not being the original boot source, and there I see the contents of the SD card and the the wpa_supplicant.conf that I was looking for.