sudo vi /usr/share/libvirt/cpu_map.xml
以下を追加

    <model name='Ryzen'>
      <signature family='23' model='1'/>
      <vendor name='AMD'/>
      <feature name='3dnowprefetch'/>
      <feature name='abm'/>
      <feature name='adx'/>
      <feature name='aes'/>
      <feature name='apic'/>
      <feature name='avx'/>
      <feature name='avx2'/>
      <feature name='bmi1'/>
      <feature name='bmi2'/>
      <feature name='clflush'/>
      <feature name='cmov'/>
      <feature name='cx16'/>
      <feature name='cx8'/>
      <feature name='de'/>
      <feature name='f16c'/>
      <feature name='fma'/>
      <feature name='fpu'/>
      <feature name='fxsr'/>
      <feature name='fsgsbase'/>
      <feature name='lahf_lm'/>
      <feature name='lm'/>
      <feature name='mca'/>
      <feature name='mce'/>
      <feature name='misalignsse'/>
      <feature name='movbe'/>
      <feature name='mmx'/>
      <feature name='msr'/>
      <feature name='mtrr'/>
      <feature name='nx'/>
      <feature name='pae'/>
      <feature name='pat'/>
      <feature name='pclmuldq'/>
      <feature name='pdpe1gb'/>
      <feature name='pge'/>
      <feature name='pni'/>
      <feature name='popcnt'/>
      <feature name='pse'/>
      <feature name='pse36'/>
      <feature name='rdtscp'/>
      <feature name='rdrand'/>
      <feature name='rdseed'/>
      <feature name='sep'/>
      <feature name='smap'/>
      <feature name='smep'/>
      <feature name='sse'/>
      <feature name='sse2'/>
      <feature name='sse4.1'/>
      <feature name='sse4.2'/>
      <feature name='sse4a'/>
      <feature name='ssse3'/>
      <feature name='svm'/>
      <feature name='syscall'/>
      <feature name='tsc'/>
      <feature name='xsave'/>
      <feature name='xsaveopt'/>
      <feature name='xsavec'/>
      <feature name='xsavec'/>
      <feature name='xgetbv1'/>
    </model>

「実行結果」
qemu-system-x86_64 -enable-kvm -name guest=ubuntu17.04,debug-threads=on -S -object secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-1-ubuntu17.04/master-key.aes -machine pc-i440fx-zesty,accel=kvm,usb=off,dump-guest-core=off -cpu Opteron_G5,+movbe,+rdrand,+fsgsbase,+bmi1,+avx2,+smep,+bmi2,+rdseed,+adx,+smap,+xsaveopt,+xsavec,+xgetbv1,-xop,-fma4,-tbm -m 4048 -realtime mlock=off -smp 4,sockets=4,cores=1,threads=1 -uuid c7f2c7db-a212-4ee7-a8cf-78a3dbc826ba -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/domain-1-ubuntu17.04/monitor.sock,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc,driftfix=slew -global kvm-pit.lost_tick_policy=discard -no-hpet -no-shutdown -global PIIX4_PM.disable_s3=1 -global PIIX4_PM.disable_s4=1 -boot strict=on -device ich9-usb-ehci1,id=usb,bus=pci.0,addr=0x5.0x7 -device ich9-usb-uhci1,masterbus=usb.0,firstport=0,bus=pci.0,multifunction=on,addr=0x5 -device ich9-usb-uhci2,masterbus=usb.0,firstport=2,bus=pci.0,addr=0x5.0x1 -device ich9-usb-uhci3,masterbus=usb.0,firstport=4,bus=pci.0,addr=0x5.0x2 -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x6 -drive file=/var/lib/libvirt/images/ubuntu17.04.qcow2,format=qcow2,if=none,id=drive-virtio-disk0 -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x7,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -drive if=none,id=drive-ide0-0-0,readonly=on -device ide-cd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0 -netdev tap,fd=26,id=hostnet0,vhost=on,vhostfd=28 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:af:b4:45,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -chardev spicevmc,id=charchannel0,name=vdagent -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=com.redhat.spice.0 -device usb-tablet,id=input0,bus=usb.0,port=1 -vnc 127.0.0.1:0 -device qxl-vga,id=video0,ram_size=67108864,vram_size=67108864,vram64_size_mb=0,vgamem_mb=16,max_outputs=1,bus=pci.0,addr=0x2 -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -chardev spicevmc,id=charredir0,name=usbredir -device usb-redir,chardev=charredir0,id=redir0,bus=usb.0,port=2 -chardev spicevmc,id=charredir1,name=usbredir -device usb-redir,chardev=charredir1,id=redir1,bus=usb.0,port=3 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x8 -msg timestamp=on