qapi: Rename 'dirty-bitmap' mode to 'incremental'

qapi: Rename 'dirty-bitmap' mode to 'incremental'

If we wish to make differential backups a feature that's easy to access,
it might be pertinent to rename the "dirty-bitmap" mode to "incremental"
to make it clear what /type/ of backup the dirty-bitmap is helping us
perform.

This is an API breaking change, but 2.4 has not yet gone live,
so we have this flexibility.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1433463642-21840-2-git-send-email-jsnow@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>