qemu-doc: Move appendix "Deprecated features" to its own file

qemu-doc: Move appendix "Deprecated features" to its own file

Consumers of QEMU need to track feature deprecation.  Keeping
deprecation documentation in its own file helps in two small ways:

* You can track changes the easy and obvious way, with git-log.
  Before, you had to resort to more complex gittery like "git-log
  --oneline -L '/@node Deprecated features/,/@node Supported build
  platforms/:qemu-doc.texi'"

* It lets us use MAINTAINERS to copy interested parties on deprecation
  patches, so they can advise or object before they're a done deal.
  The next commit will do that for libvirt.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180716073226.21127-2-armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>