[Libguestfs] [PATCH nbdkit v2 0/3] Access export name from plugins.

The previous incomplete patch was here: https://www.redhat.com/archives/libguestfs/2019-September/msg00049.html based on earlier discussion here: https://www.redhat.com/archives/libguestfs/2019-September/msg00047.html In v2: - The previous patch was incomplete. This version completes it by adding tests and extending nbdkit-sh-plugin. - nbdkit_export_name now returns NULL for error, setting nbdkit_error. The only possible error currently is if we're not called from a connected callback. - For oldstyle protocol, "" is returned. - Add some clarifying text to docs for can_multi_conn. - Use strcpy in one place instead of memcpy (but leave the other one alone). As before this patch doesn't attempt to resolve any problems with our current handling of -e, NBD_OPT_LIST, etc. Rich.