exec: call cpu_exec_exit() from a CPU unrealize common function

As cpu_exec_exit() mirrors the cpu_exec_realizefn(),
rename it as cpu_exec_unrealizefn().

Create and register a cpu_common_unrealizefn() function for
the CPU device class and call cpu_exec_unrealizefn() from
this function.

Remove cpu_exec_exit() from cpu_common_finalize()
(which mirrors init, not realize), and as x86_cpu_unrealizefn()
and ppc_cpu_unrealizefn() overwrite the device class unrealize function,
add a call to a parent_unrealize pointer.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>