docs: document deprecation policy & deprecated features in appendix

The deprecation of features in QEMU is totally adhoc currently,
with no way for the user to get a list of what is deprecated
in each release. This adds an appendix to the doc that records
when each deprecation was made and provides text explaining
what to use instead, if anything.

Since there has been no formal policy around removal of deprecated
features in the past, any deprecations prior to 2.10.0 are to be
treated as if they had been made at the 2.10.0 release. Thus the
earliest that existing deprecations will be deleted is the start
of the 2.12.0 cycle.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20170725113638.7019-1-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>