Description of problem:
In Openshift Virtualization, when we try to set passt interface MTU explicitly libvirt fail with the following error:

virError(Code=67, Domain=20, Message='unsupported configuration: unmanaged target dev is not supported on interfaces of type 'user'')

The domain interface XML spec set is as follow:

<interface type="user">
<source dev="eth0"></source>
<target dev="eth0" managed="no"></target>
<model type="virtio-non-transitional"></model>
<mtu size="1480"></mtu>
<alias name="ua-testPasst"></alias>
<backend type="passt" logFile="/var/run/kubevirt/passt.log"></backend>
<portForward proto="tcp"></portForward>
<portForward proto="udp"></portForward>
</interface>

Version-Release number of selected component (if applicable):
libvirt version: 9.5.0, package: 6.el9 (builder@centos.org, 2023-08-25-08:53:56, )
passt version: 0^20230818.g0af928e-4.el9.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Define a domain with passt interface, set to non managed, and explicit MTU.

Actual results:
Domain failed to start.

Expected results:
Domain with passt interface, not managed by libvirt, with explicit MTU work.

Additional info: