To set a custom resolution > 1920x1080 in a OSX-KVM
VM, edit the libvirt xml with:
<video>
<model type="vga" vram="32768" heads="1" primary="yes">
<resolution x="3440" y="1440"/>
</model>
<alias name="video0"/>
<address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>