virsh-snap.sh: create and revert external libvirt snapshots easily

There are two types of disk snapshots in libvirt: internal and external. Internal snapshots (virsh default) store all data in one file. With external snapshots, base data and snapshot data is stored separately: base disk file is left intact, and all changes are written into a separate snapshot overlay file. For me, this is the way 😉 Unfortunately, external snapshots support in virsh is limited, virt-manager GUI can't create / use them, and reverting external snapshots involves manual editing config xml to change disk file path.

The manual xml editing part was what irritated me most, so I figured I'd just save VM config with virsh dumpxml before creating a snapshot, and then restore it with virsh define if / when I'd need to revert. Then I decided to wrap this and other routine commands needed to work with external snapshots into a script, virsh-snap.sh.

Installation

Download script:

wget -P ~/.local/bin https://github.com/shvchk/virsh-external-snapshots/raw/main/virsh-snap.sh

Make it executable:
```
chmod +x ~/.local/bin/virsh-snap.sh
```

Usage

Create snapshot named snapshot-1 of vm-42:
```
virsh-snap.sh create vm-42 snapshot-1
```
Snapshot name is optional, by default current date and time is used as a name, e.g. 2022.07.29_23.55.12.

List snapshots of vm-42:
```
virsh-snap.sh list vm-42
```
Note that this is not the same as virsh snapshot-list. Snapshots are created without metadata and won't be shown with virsh snapshot-list.

Revert and delete snapshot-1 of vm-42:
```
virsh-snap.sh revert vm-42 snapshot-1
```

Revert, but don't delete snapshot-1 of vm-42:
```
virsh-snap.sh soft-revert vm-42 snapshot-1
```
This allows to use this snapshot later with unrevert.

Undo revert of a snapshot-1 of vm-42, i.e. make it active again:
```
virsh-snap.sh unrevert vm-42 snapshot-1
```

Delete snapshot snapshot-1 of vm-42:
```
virsh-snap.sh delete vm-42 snapshot-1
```
Might be useful if you change your mind after soft-revert.

Show help:

virsh-snap.sh help

virsh-snap.sh: create and revert external libvirt snapshots easily

Usage: virsh-snap.sh <action> <domain> [snapshot name]


Actions:
--------
create, c           Create snapshot
                    If no snapshot name is provided, current date and time is used

list, ls, l         List snapshots

disk                Show active disk

revert, rev, r      Revert snapshot and delete it (same as soft-revert + delete)

soft-revert,
srev, sr            Revert snapshot without deleting it (allows unrevert)

unrevert,
unrev, ur           Unrevert snapshot, i.e. make soft-reverted snapshot active again

delete, del, rm     Delete snapshot

help, h             This message


Snapshots are created without metadata, so they won't be shown with `virsh snapshot-list`