btrfs improvements and fixes

[mgziminsky]

A few small fixes and improvements to btrfs partitioning to better support manual layouts.

• Fix: Don't require subvolume mount points to exist on host system
• Fix: Mount subvolumes in sorted order to properly support nesting
• Enhance: Support deeply nested subvolume names as allowed by btrfs

Tests and Checks

• I have tested the code!
  • Tested and verified in qemu using the example config below

Example Config

{
    "config_version": "3.0.1",
    "disk_config": {
        "config_type": "manual_partitioning",
        "device_modifications": [
            {
                "device": "/dev/sda",
                "partitions": [
                    {
                        "btrfs": [],
                        "dev_path": null,
                        "flags": [
                            "boot",
                            "esp"
                        ],
                        "fs_type": "fat32",
                        "mount_options": [],
                        "mountpoint": "/boot",
                        "obj_id": "415b73ac-e232-4dba-b5fa-04691ab05b1c",
                        "size": {
                            "sector_size": {
                                "unit": "B",
                                "value": 512
                            },
                            "unit": "GiB",
                            "value": 1
                        },
                        "start": {
                            "sector_size": {
                                "unit": "B",
                                "value": 512
                            },
                            "unit": "MiB",
                            "value": 1
                        },
                        "status": "create",
                        "type": "primary"
                    },
                    {
                        "btrfs": [
                            {
                                "mountpoint": "/var/log",
                                "name": "@log"
                            },
                            {
                                "mountpoint": "/var/cache",
                                "name": "@cache"
                            },
                            {
                                "mountpoint": "/",
                                "name": "@/live/snapshot"
                            },
                            {
                                "mountpoint": "/.snapshots",
                                "name": "@"
                            },
                            {
                                "mountpoint": "/home",
                                "name": "@home/live/snapshot"
                            },
                            {
                                "mountpoint": "/home/.snapshots",
                                "name": "@home"
                            }
                        ],
                        "dev_path": null,
                        "flags": [],
                        "fs_type": "btrfs",
                        "mount_options": [
                            "compress=zstd"
                        ],
                        "mountpoint": null,
                        "obj_id": "824a7f4e-29a3-4414-a425-e575ba8861d4",
                        "size": {
                            "sector_size": {
                                "unit": "B",
                                "value": 512
                            },
                            "unit": "B",
                            "value": 33283899392
                        },
                        "start": {
                            "sector_size": {
                                "unit": "B",
                                "value": 512
                            },
                            "unit": "B",
                            "value": 1074790400
                        },
                        "status": "create",
                        "type": "primary"
                    }
                ],
                "wipe": true
            }
        ]
    },
    "version": "3.0.1"
}

[svartkanin]

Thanks for the improvements