setting guest mtu

[Marc Roos]

I was trying to see if it is possible to automatically set the mtu size 
for the guest. I am using a tunnel with smaller mtu so when I switch 
over vm's, it would be nice if they get this tunnel mtu automatically. 

This config:
    <interface type='direct'>
      <source dev='tun1' mode='bridge'/>
      <model type='virtio'/>
      <mtu size='1462'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' 
function='0x0'/>
    </interface>

Generates this error:
error: unsupported configuration: setting MTU on interface type direct 
is not supported yet

Is there another way to set this, like with this host_mtu as mentioned 
in[1], or can I forget about trying? this?

qemu-system-x86-2.0.0-1.el7.6.x86_64
qemu-kvm-1.5.3-173.el7_8.3.x86_64
libvirt-4.5.0-33.el7_8.1.x86_64

[1]
https://www.mail-archive.com/libvirt-users@redhat.com/msg10746.html