Start virtual machines in paused state if offline to allow full/incremental/differential backup

[abbbi]

Currently the backup utility performs an fallback to copy backup if a virtual machine is offline because its not possible to create checkpoints for offline virtual machines (no qemu process running ..)

Libvirt allows to start the virtual machine in paused state, where the qemu process for the virtual machine is executed but the CPU's of the virtual machine are halted. The Virtual domain is then in "paused" state. However, as the qemu process is running, backup and checkpoint operations are possible.

A special option -S (--start-domain) could be introduced which starts the domain with PAUSE and AUTODESTROY Flags: it is then possible to perform full/inc/diff backups on virtual machines that are offline, too.

[abbbi]

Branch contains first draft of implementation where it is attempted to start the virtual machine if option is specified:

 ./virtnbdbackup -l inc -d vm1 -o /tmp/paused -S
[2024-03-21 21:33:04] INFO lib common - printVersion [main]:  Version: 2.2 Arguments: ./virtnbdbackup -l inc -d vm1 -o /tmp/paused -S
[2024-03-21 21:33:04] INFO root virtnbdbackup - main [main]:  Backup level: [inc]
[2024-03-21 21:33:05] INFO root virtnbdbackup - main [main]:  Libvirt library version: [9000000]
[2024-03-21 21:33:05] INFO root virtnbdbackup - main [main]:  Starting offline VM in paused state
[..]

Still TODO:

• think about possible corner cases where mixing option -S and fallback could lead to data loss?
• update README
• add tests