Rename VM `security.csm` setting to indicate LXD no longer provides UEFI CSM mode

[tomponline]

Please confirm

• I have searched existing issues to check if an issue already exists for the bug I encountered.

Distribution

ubuntu

Distribution version

24.04

Output of "snap list --all lxd core20 core22 core24 snapd"

N/A

Output of "lxc info" or system info if it fails

N/A

Issue description

Originally security.csm=true enabled CSM mode in EDK2, however we still had issues booting legacy guest VMs and development of the CSM mode in EDK2 upstream ceased shortly afterwards.

So currently security.csm=true enabled Seabios instead and doesn't use EDK2 CSM.

We could rename this setting to something like boot.mode which would accept either "uefi" (the default, using EDK2) or "bios" (which would use seabios).

Steps to reproduce

N/A

Information to attach

• Any relevant kernel output (dmesg)
• Instance log (lxc info NAME --show-log)
• Instance configuration (lxc config show NAME --expanded)
• Main daemon log (at /var/log/lxd/lxd.log or /var/snap/lxd/common/lxd/logs/lxd.log)
• Output of the client with --debug
• Output of the daemon with --debug (or use lxc monitor while reproducing the issue)