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I compiled a new kernel for my KVM client, and the ethernet interface is not recognized

you can tell qemu to emulate e.g. the 8139 with model=rtl8139

I can't ping from host to guest or vice versa. How can I access the host please?

ping does not go through, but I suppose you can access the host in some way, i.e. you can navigate for sure internet(e.g. firefox) if host can