cpu: call AccelCPUClass::cpu_realizefn in cpu_exec_realizefn

move the call to accel_cpu->cpu_realizefn to the general
cpu_exec_realizefn from target/i386, so it does not need to be
called for every target explicitly as we enable more targets.

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: <20210322132800.7470-6-cfontana@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>