cpu: Call plugin hooks only when ready

The initialization and exit hooks will not affect the state of vCPU
outside TCG context, but they may depend on the state of vCPU.
Therefore, it's better to call plugin hooks after the vCPU state is
fully initialized and before it gets uninitialized.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231025093128.33116-16-akihiko.odaki@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231106185112.2755262-12-alex.bennee@linaro.org>