nbd-client: Support leading / in NBD URI

The NBD URI specification [1] states that only one leading slash at
the beginning of the URI path component is stripped, not all such
slashes.  This becomes important to a patch I just proposed to nbdkit
[2], which would allow the exportname to select a file embedded within
an ext2 image: ext2fs demands an absolute pathname beginning with '/',
and because qemu was inadvertantly stripping it, my nbdkit patch had
to work around the behavior.

[1] https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.md
[2] https://www.redhat.com/archives/libguestfs/2020-February/msg00109.html

Note that the qemu bug only affects handling of URIs such as
nbd://host:port//abs/path (where '/abs/path' should be the export
name); it is still possible to use --image-opts and pass the desired
export name with a leading slash directly through JSON even without
this patch.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200212023101.1162686-1-eblake@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

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