target/i386: move accel_cpu_instance_init to .instance_init

target/i386: move accel_cpu_instance_init to .instance_init

With the reordering of instance_post_init callbacks that is new in 10.1
accel_cpu_instance_init must execute in .instance_init as is already
the case for RISC-V.  Otherwise, for example, setting the vendor
property is broken when using KVM or Hypervisor.framework, because
KVM sets it *after* the user's value is set by DeviceState's
intance_post_init callback.

Reported-by: Like Xu <like.xu.linux@gmail.com>
Reported-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>