[Libguestfs] [PATCH nbdinfo proposal] info: Add a --map option for displaying allocation metadata

This is a rough-and-ready implementation of nbdinfo --map option, to display the allocation and other metadata of NBD exports. I only tested it lightly against nbdkit. It needs at least some tests. Command like these should work: nbdinfo --map nbd://localhost nbdinfo --map=qemu:dirty-bitmap nbd://localhost Rich.