fw_cfg support for qemu module

[LaszloGombos]

Enhance the qemu dracut module so that it can extend the command line that was passed in when Linux kernel started.

This can happen either using the fw_cfg or smbios qemu arguments.

• -fw_cfg name=opt/io.dracut.cmdline-extra,string=ro
• -smbios type=11,value=io.dracut.cmdline-extra:ro

See https://www.qemu.org/docs/master/specs/fw_cfg.html and https://www.qemu.org/docs/master/system/invocation.html#hxtool-4

systemd has similar feature for credential management - https://github.com/systemd/systemd/blob/main/docs/CREDENTIALS.md?plain=1#L53

When qemu is used with UKI (and not Direct Linux Boot), -append command line option is not available to control kernel command line arguments.

To read the value from within the guest passed in via qemu cat /sys/firmware/qemu_fw_cfg/by_name/opt/io.dracut.cmdline-extra/raw