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

[Richard W.M. Jones]

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.