Replace functionality for offline backup with -S option, make -S option default

[abbbi]

At the moment there are two ways for backing up offline virtual machines:

1. Use the offline backup functionality in virtnbdbackup where the code starts the NBD Service using qemu-nbd, only copy backup works with this mode
2. Use option -S which starts VM in non-operational mode and enables all backup modes to work correctly.

I think the situations where option 1) is the better way for backing up the virtual machine are very limited. I cant think of situations where the virtual machine is not able to be started in on-operational mode.

It may make sense to enable option -S by default and remove all the legacy code from the codebase that handles starting the local and remote qemu-nbd processes for the offline backup.