So I want to stop this terrible increase in the size of the 'userdata-qemu.img.qcow2' file of my virtual machine. Is there a solution for this?

It's strange, but it's ok to delete userdata.img and userdata-qemu.img. But emulator can't run without the last file, but it can be empty! So make empty file userdata-qemu.img (for example, using touch command in terminal), and that's it. I removed 6,5GB + 0,5GB for just one of virtual devices.