avoid open coding the accesses to cpu->accel_cpu interfaces,
and instead introduce:

accel_cpu_instance_init,
accel_cpu_realizefn

to be used by the targets/ initfn code,
and by cpu_exec_realizefn respectively.

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: <20210322132800.7470-7-cfontana@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>