i386/cpu: Move x86_ext_save_areas[] initialization to .instance_init

i386/cpu: Move x86_ext_save_areas[] initialization to .instance_init

In x86_cpu_post_initfn(), the initialization of x86_ext_save_areas[]
marks the unsupported xsave areas based on Host support.

This step must be done before accel_cpu_instance_init(), otherwise,
KVM's assertion on host xsave support would fail:

qemu-system-x86_64: ../target/i386/kvm/kvm-cpu.c:149:
kvm_cpu_xsave_init: Assertion `esa->size == eax' failed.

(on AMD EPYC 7302 16-Core Processor)

Move x86_ext_save_areas[] initialization to .instance_init and place it
before accel_cpu_instance_init().

Fixes: commit 5f158abef44c ("target/i386: move accel_cpu_instance_init to .instance_init")
Reported-by: Paolo Abeni <pabeni@redhat.com>
Tested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250717023933.2502109-1-zhao1.liu@intel.com
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>