vm/qemu: use virtio-net-ccw as virtual netdev on s390x arch #591

Job	Run time
build	1m 4s
aux	1m 3s
dashboard	1s
race	1s
race_dashboard	1s
arch (presubmit_arch_linux)	1m 1s
arch (presubmit_arch_freebsd)	1m 2s
arch (presubmit_arch_netbsd)	1m 3s
gvisor	1m 2s
arch (presubmit_arch_openbsd)	1m 3s
arch (presubmit_arch_darwin)	1m 3s
old	1m 8s
arch (presubmit_arch_windows)	1m 3s
arch (presubmit_arch_executor)	1m 3s
	11m 38s

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