>  HTTP POST https://[fd2e:6f44:5dd8::1]:8000/redfish/v1/Managers/e46202f9-5647-4188-a0ef-3e520a061ac2/VirtualMedia/Cd/Actions/VirtualMedia.InsertMedia returned code 500. Base.1.0.GeneralError: Failed fetching image from URL http://[fd00:1101:0:1::3]:6180/redfish/boot-91499985-2979-44d8-894a-fc0c738de7c9.iso?filename=tmp71_k6yzg.iso: HTTPConnectionPool(host='fd00:1101:0:1::3', port=6180): Max retries exceeded with url: /redfish/boot-91499985-2979-44d8-894a-fc0c738de7c9.iso?filename=tmp71_k6yzg.iso (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7fb2cd47b6d8>: Failed to establish a new connection: [Errno 101] Network is unreachable',))

This error suggests that the BMC (in your case sushy-tools?) isn't able to contact the masters IPv6 address on the provisioning network, 

I was able to reproduce this with the instructions you provided, in my case the problem was that my provisioning bridge on the virt host didn't have an IP on the fd00:1101:0:1::/64 range, It did have one on the 
fd00:1101::/64 range (the dev-scripts default) but your not using the default range.

I'm guessing the system your using to setup the virt infrastructure isn't setting up the provisioning bridge with a IPv6 as its run with a provisioning network Disabled and doesn't expect it to be re-enabled after the fact. Can you see if giving your provisioning bridge an IP in the fd00:1101:0:1::/64 range solves the problem.