cpu: Introduce CPU model deprecation API

Implement the ability of marking some versions deprecated. When
that CPU model is chosen, print a warning.  The warning message
can be customized, e.g. suggesting an alternative CPU model to be
used instead.

The deprecation message will be printed by x86_cpu_list_entry(),
e.g. '-cpu help'.

QMP command 'query-cpu-definitions' will return a bool value
indicating the deprecation status.

Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
Message-Id: <1600758855-80046-1-git-send-email-robert.hu@linux.intel.com>
[ehabkost: reword commit message]
[ehabkost: Handle NULL cpu_type]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>