xen-tools autopkgtest cannot handle deb822 sources

[Nick Rosbrook]

Part of the 'funcionality' autopkgtest in xen-tools parses sources.list to try and find a useful source entry. This is failing on noble with[1]:

905s autopkgtest [12:00:54]: test functionality: - - - - - - - - - - results - - - - - - - - - -
905s functionality FAIL stderr: Couldn't find a useful entry in the sources.list files of the Dom0. Tried:
906s autopkgtest [12:00:55]: test functionality: - - - - - - - - - - stderr - - - - - - - - - -
906s Couldn't find a useful entry in the sources.list files of the Dom0. Tried:
906s /etc/apt/sources.list
906s Couldn't find a useful entry in the sources.list files of the Dom0. Tried:
906s /etc/apt/sources.list
906s Couldn't find a useful entry in the sources.list files of the Dom0. Tried:
906s /etc/apt/sources.list
906s Couldn't find a useful entry in the sources.list files of the Dom0. Tried:
906s /etc/apt/sources.list
906s Couldn't find a useful entry in the sources.list files of the Dom0. Tried:
906s /etc/apt/sources.list
906s Couldn't find a useful entry in the sources.list files of the Dom0. Tried:
906s /etc/apt/sources.list
906s Couldn't find a useful entry in the sources.list files of the Dom0. Tried:
906s /etc/apt/sources.list

This is probably related to the fact that Ubuntu moved to deb822 sources by default in noble, and the default sources are now located at /etc/apt/sources.list.d/ubuntu.sources.

[1] https://autopkgtest.ubuntu.com/results/autopkgtest-noble/noble/amd64/x/xen-tools/20240415_121306_b4652@/log.gz