kvm "default" NAT network doesn't work

You need to setup a macvtap from your host to allow your VM's guest to have an internet connection which in your case you have your NAT configuration for hosts to guest connection only.

Also, I advised you to have virt-manager installed to know the basic concept and setups of KVM graphically then you can do it to CLI the next time.

For your reference,I have a connection from host to guest connection this configuration is NAT with virtio.

Now for your guest VM to have a connection set up the macvtap, but first you have to create a separate bridge connection here and then the macvtap.

By the time you set it up you should have this network configuration.

Another one would be the guest xml file which was located in /etc/libvirt/qemu/guest.xml - But do not edit that it only serves as reference

I am not adept on the CLI part but here is the thing I've done to make this case work.

Hope this helps.