block/copy-before-write: add on-cbw-error open parameter

Currently, behavior on copy-before-write operation failure is simple:
report error to the guest.

Let's implement alternative behavior: break the whole copy-before-write
process (and corresponding backup job or NBD client) but keep guest
working. It's needed if we consider guest stability as more important.

The realisation is simple: on copy-before-write failure we set
s->snapshot_ret and continue guest operations. s->snapshot_ret being
set will lead to all further snapshot API requests. Note that all
in-flight snapshot-API requests may still success: we do wait for them
on BREAK_SNAPSHOT-failure path in cbw_do_copy_before_write().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>