[s390x][svirt] Print content of kernel log and serial console into "Serial output" field
QEMU backend prints output of kernel logs (serial0 file made by QEMU itself, saved as serial.txt) and virtio console (virtio_console.log made by QEMU itself and then saved as serial_terminal.txt) into "Serial output (serial0.txt and serial_terminal.txt)" box.
But on svirt kernel logs (serial0 created by output of SSH connection, saved as serial.txt) goes into "Live log" box, where is mixed with openQA log autoinst-log.txt.
And #45863 (https://github.com/os-autoinst/os-autoinst/pull/1109) is going to continue in this trend.
To fix this, #46967 might need to be fixed.