Move to xml mappings

Depends on https://github.com/sonata-project/SonataAdminBundle/pull/1472 - merged.

Afterwards, this PR can be merged https://github.com/symfony-cmf/cmf-sandbox/pull/191

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony-cmf/BlockBundle/issues/85
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/199