i386/cpu: Mark APX xstate as migratable

APX xstate is user xstate. The related registers are cached in
X86CPUState. And there's a vmsd "vmstate_apx" to migrate these
registers.

Thus, it's safe to mark it as migratable.

Tested-by: Xudong Hao <xudong.hao@intel.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20251211070942.3612547-10-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/cpu.c

@@ -1544,7 +1544,8 @@ FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
         .migratable_flags = XSTATE_FP_MASK | XSTATE_SSE_MASK |
             XSTATE_YMM_MASK | XSTATE_BNDREGS_MASK | XSTATE_BNDCSR_MASK |
             XSTATE_OPMASK_MASK | XSTATE_ZMM_Hi256_MASK | XSTATE_Hi16_ZMM_MASK |
-            XSTATE_PKRU_MASK | XSTATE_XTILE_CFG_MASK | XSTATE_XTILE_DATA_MASK,
+            XSTATE_PKRU_MASK | XSTATE_XTILE_CFG_MASK | XSTATE_XTILE_DATA_MASK |
+            XSTATE_APX_MASK,
     },
     [FEAT_XSAVE_XCR0_HI] = {
         .type = CPUID_FEATURE_WORD,