Move invocation of CPUClass.cpu_exec_*() to separate helpers,
to make it easier to refactor that code later.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Claudio Fontana <cfontana@suse.de>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201212155530.23098-10-cfontana@suse.de>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>