nbd: Add qemu-nbd -D for human-readable description

The NBD protocol allows servers to advertise a human-readable
description alongside an export name during NBD_OPT_LIST.  Add
an option to pass through the user's string to the NBD client.

Doing this also makes it easier to test commit 200650d4, which
is the client counterpart of receiving the description.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1476469998-28592-2-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/qemu-nbd.texi b/qemu-nbd.texi
index b7a9c6d..9a84e81 100644
--- a/qemu-nbd.texi
+++ b/qemu-nbd.texi
@@ -79,9 +79,12 @@
 Allow up to @var{num} clients to share the device (default @samp{1})
 @item -t, --persistent
 Don't exit on the last connection
-@item -x NAME, --export-name=NAME
+@item -x, --export-name=@var{name}
 Set the NBD volume export name. This switches the server to use
 the new style NBD protocol negotiation
+@item -D, --description=@var{description}
+Set the NBD volume export description, as a human-readable
+string. Requires the use of @option{-x}
 @item --tls-creds=ID
 Enable mandatory TLS encryption for the server by setting the ID
 of the TLS credentials object previously created with the --object