VHD: differencing disk can be opened with no parent, fix by dotnet4fun [#5281]
VHD: differencing disk can be opened with no parent, fix by dotnet4fun [#5281]

Note the filesystem is likely corrupt, so opened disk is really only of use for
block-level access.

breezechen/DiscUtils,quamotion/discutils,drebrez/DiscUtils,breezechen/DiscUtils