blockdev-backup: Add error handling option for copy-before-write jobs

blockdev-backup: Add error handling option for copy-before-write jobs

This patch extends the blockdev-backup QMP command to allow users to specify
how to behave when IO errors occur during copy-before-write operations.
Previously, the behavior was fixed and could not be controlled by the user.

The new 'on-cbw-error' option can be set to one of two values:
- 'break-guest-write': Forwards the IO error to the guest and triggers
  the on-source-error policy. This preserves snapshot integrity at the
  expense of guest IO operations.
- 'break-snapshot': Allows the guest OS to continue running normally,
  but invalidates the snapshot and aborts related jobs. This prioritizes
  guest operation over backup consistency.

This enhancement provides more flexibility for backup operations in different
environments where requirements for guest availability versus backup
consistency may vary.

The default behavior remains unchanged to maintain backward compatibility.

Signed-off-by: Raman Dzehtsiar <Raman.Dzehtsiar@gmail.com>
Message-ID: <20250414090025.828660-1-Raman.Dzehtsiar@gmail.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
[vsementsov: fix long lines]
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Tested-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>