hvf: ubuntu iso boot menu issue

With hvf acceleration on macOS, ubuntu server installation ISO boot language menu shows fractured images.

To reproduce the issue:
./x86_64-softmmu/qemu-system-x86_64 -m 800 -accel hvf -cdrom ~/ubuntu-16.04.4-server-amd64.iso

Control:
./x86_64-softmmu/qemu-system-x86_64 -m 800 -accel tcg -cdrom ~/ubuntu-16.04.4-server-amd64.iso

Host: macOS Mojave 10.14.3
Guest: Ubuntu Server 16.04.4 ISO
QEMU: version 3.1.94 (v4.0.0-rc4)