So the qemu addon was updated from version 7.1.0 to 8.0.3 in IPFire Core Update 179.
Searching on your error message this is produced when the slirp library is not available and if nothing different is specified then the default network backend type is slirp.
In qemu-7.2 they removed the bundled slirp library and it has to be installed as its own separate library. That library is required for the net user.
Unfortunately no one evaluated the qemu addon update in Core Update 179 Testing so this was not picked up.
I will do a new update patch for qemu tomorrow that includes the slirp library.
Updated patch has been submitted to the IPFire dev mailing list. There is enough time that it will get merged into CU180 Testing.
I could not test the changes I made as I don’t use qemu in IPFire. Hopefully someone will be able to test it when it has been merged before Core Update 180 is released to confirm that everything now works.