libguestfs: missing disk format specifier when adding a disk

libguestfs before 1.5.23, as used in virt-v2v, virt-inspector 1.5.3 and earlier, and possibly other products, when a raw-format disk image is used, allows local guest OS administrators to read files from the host via a crafted 1 qcow2, 2 VMDK, or 3 VDI header, related to lack of support for a dis...