Re: [Libguestfs] [PATCH] lib: create: Allow any [[:alnum:]]+ string as a backingfmt parameter.

If you use the libguestfs tools which open disk images read-only (eg. virt-df), with formats such as 'vdi', then you will see an error: error: invalid value for backingformat parameter 'vdi' This is because opening a disk image read-only will try to create a qcow2 file with the original image as a backing file. However the list of permitted backing formats was very restrictive and did not include 'vdi' (nor many other uncommon formats). Instead of using a whitelist for backing formats, just validate that the string is alphanumeric and short. Thanks: Mike Goodwin for reporting the bug. ---