[Libguestfs] Few libnbd questions/concerns

Here are few things I found out when using libnbd that might be perfectly fine or maybe just an oversight, but I wanted to point them out. It's nothing major. When running a program with `nbdkit -U - --run ...`, the $nbd parameter gets expanded to nbd:unix:/path/to/socket. When this string is passed to nbd_connect_uri(), it does not return an error (even though it is not a valid URL), but what's more it treats it as "nbd://localhost", which might be a completely different server (that actually happened to me and it was kind of confusing). When nbd_block_status() callback returns -1, the nbd_block_status() function reports: command failed: Protocol error (errno = 71) which is a bit confusing to me. I could be nicer to have that report that it really was the callback that caused this. One last thing is that I could not find any definition for the flags of "base:allocation" metadata (NBD_STATE_HOLE and NBD_STATE_ZERO). It might just be things that are still missing due to an early stage of libnbd, or it might not belong there or it might be me misusing something. Have a nice day, Martin