Modern way is using blockdev-add + blockdev-backup, which provides a
lot more control on how target is opened.

As example of drive-backup problems consider the following:

User of drive-backup expects that target will be opened in the same
cache and aio mode as source. Corresponding logic is in
drive_backup_prepare(), where we take bs->open_flags of source.

It works rather bad if source was added by blockdev-add. Assume source
is qcow2 image. On blockdev-add we should specify aio and cache options
for file child of qcow2 node. What happens next:

drive_backup_prepare() looks at bs->open_flags of qcow2 source node.
But there no BDRV_O_NOCAHE neither BDRV_O_NATIVE_AIO: BDRV_O_NOCAHE is
places in bs->file->bs->open_flags, and BDRV_O_NATIVE_AIO is nowhere,
as file-posix parse options and simply set s->use_linux_aio.

The documentation is updated in a minimal way, so that drive-backup is
noted only as a deprecated command, and blockdev-backup used in most of
places.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>