[svirt][serial][xen] Enable svirt serial console for XEN
Svirt serial console has been enabled only for s390x, used mainly for LTP tests.
It'd be good to enable it for other archs, let's start with XEN as it was requested.
See PR, which needs debugging and finding valid libvirt code part
Fixes for original implementation for s390x (#45863)
Some related PR to understand the code a bit
https://github.com/os-autoinst/os-autoinst/pull/1154 (logging xml file in autoinst-log.txt)
NOTE: it's not working on Hyper-V and VMware either.