What were you trying to do that didn't work?

Start guest failed when the guest has network image as the data file

Please provide the package NVR for which the bug is seen:

libvirt-10.10.0-1.el10.x86_64
qemu-kvm-9.1.0-7.el10.x86_64

How reproducible is this bug?:

100%

Steps to reproduce

1.Prepare a file image which use the nbd network image as its data file.

# setenforce 0   (allow qemu to access the socket)
# mkdir -p /var/run/nbd-server
# systemd-run qemu-nbd --socket=/var/run/nbd-server/datafile --export-name=datafile --persistent /var/lib/libvirt/images/datastore_nbd
# chown qemu:qemu /var/run/nbd-server/datafile
# qemu-img create -f qcow2 -o data_file=nbd+unix:///datafile?socket=/var/run/nbd-server/datafile /var/lib/libvirt/images/datastore_nbd.qcow2 500M  

2. Prepare the guest with the following disk xml.

<disk type="file" device="disk">
  <driver name="qemu" type="qcow2"/>
  <source file="/var/lib/libvirt/images/datastore_nbd.qcow2"/>
  <target dev="vdb" bus="virtio"/>
  <address type="pci" domain="0x0000" bus="0x07" slot="0x00" function="0x0"/>
</disk>    

3. Start the guest.
# virsh start rhel
error: Failed to start domain 'rhel'
error: unsupported configuration: storage type 'dir' requires use of storage format 'fat'

Expected results

May start successfully or have clear error message, like "should provide the dataStore info for this image".

Actual results

Failed like step 3.