Deprecate the -enable-hax option

Deprecate the -enable-hax option

We currently have got three ways of turning on the HAX accelerator:
"-machine accel=hax", "-accel hax" and "-enable-hax". That's really
confusing and overloaded. Since "-accel" is our preferred way to enable
an accelerator nowadays, and "-accel hax" is even less to type than
"-enable-hax", let's deprecate the "-enable-hax" option now.

Note: While "-enable-kvm" is available since a long time and can hardly be
removed since it is used in a lot of upper layer tools and scripts, the
"-enable-hax" option is still rather new and not very widespread yet, so
I think that it should be OK if we remove this in a couple of releases again
(we'll see whether someone complains after seeing the deprecation message -
then we could still reconsider to keep it if there a well-founded reasons).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1529950933-28347-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>