I used this guide: https://github.com/xcp-ng/xcp/wiki/Create-a-local-ISO-repository
On the xen DOM0, create a place to store all ISOs, max 18gigs of space will be available.
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Then:
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Created:
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It shows up in xcp-ng center:
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Then, use git bash scp or FileZilla to sftp the files over to the dom0 /opt/mylocalisos directory, so that xen can have them.