qapi/block-core: Introduce BackupCommon

drive-backup and blockdev-backup have an awful lot of things in common
that are the same. Let's fix that.

I don't deduplicate 'target', because the semantics actually did change
between each structure. Leave that one alone so it can be documented
separately.

Where documentation was not identical, use the most up-to-date version.
For "speed", use Blockdev-Backup's version. For "sync", use
Drive-Backup's version.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
[Maintainer edit: modified commit message. --js]
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20190709232550.10724-2-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>