nbd URI wrong export name (regression in qemu 9.1)

Host environment

• Operating system: Fedora 42
• OS/kernel version: Linux 6.11.0-0.rc5.20240830git20371ba12063.47.fc42.x86_64
• Architecture: x86_64
• QEMU flavor: qemu-img
• QEMU version: qemu-img-9.1.0-2.fc42.x86_64
• QEMU command line: (see description)

Description of problem

qemu with an nbd URI seems to pass the wrong export name to the server, if the exportname is .. This seems to be a regression in qemu 9.1, because it didn't happen in 9.0.

Steps to reproduce

$ nbdkit -fv -U - null --run 'qemu-img info "nbd+unix:///.?socket=$unixsocket"'
...
nbdkit: null[1]: debug: null: open readonly=0 exportname="" tls=0

In qemu 9.0 this was correct:

nbdkit: null[1]: debug: null: open readonly=0 exportname="." tls=0