Fix edge cases around .vhd support

[benhillis]

While experimenting with the new --fixed-vhd support I realized we do not correctly handle all cases where a distro might be stored as a .vhd. This resolves those issues and clarifies usage that we support both .vhd and .vhdx.

[benhillis]

Considering adding some tests, need to convert some vhdx to vhd.

[benhillis]

Change looks good. Regarding test coverage, looks like https://learn.microsoft.com/en-us/powershell/module/hyper-v/convert-vhd?view=windowsserver2025-ps can convert from vhdx to vhd

Done.