[Libguestfs] [PATCH nbdkit] indexed-gzip: Fix restart at non-byte-aligned access points

This fixes a reproducible indexed-gzip failure when random access resumes at a non-byte-aligned access point. Without the patch, a gzip-compressed full-disk image exposes its GPT successfully, but filesystem reads fail repeatedly with |Z_DATA_ERROR (-3)|. With the patch applied to the same nbdkit source commit, the ext4 partition mounts read-only and file reads succeed. The corresponding GitLab issue is: nbdkit#32 <https://gitlab.com/nbdkit/nbdkit/-/work_items/32> A detailed unpatched-versus-patched test result is included in the commit message.