Availability of liburing in Ubuntu 20.04
I managed to make it, and here I record it as an answer for the non-conformist whose work has creative and cracking nature.
I downloaded liburing1 and liburing-dev package deb files liburing1_0.6-3ubuntu1_amd64.deb and liburing-dev_0.6-3ubuntu1_amd64.deb from another ubuntu variant. Then I use sudo dpkg -x <def file name> / to install liburing. After that, ./configure, make & make install of qemu succeeds and generated the needed qemu-system-x86_64.
I have not seen any problem so far.