target/i386: Clarify the padding requirements of X86XSaveArea

Replace the hard-coded size of offsets or structure elements with
defined constants or sizeof().

Signed-off-by: David Edmondson <david.edmondson@oracle.com>
Message-Id: <20210705104632.2902400-4-david.edmondson@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/cpu.h

@@ -1329,7 +1329,13 @@ typedef struct X86XSaveArea {
 
     /* AVX State: */
     XSaveAVX avx_state;
-    uint8_t padding[960 - 576 - sizeof(XSaveAVX)];
+
+    /* Ensure that XSaveBNDREG is properly aligned. */
+    uint8_t padding[XSAVE_BNDREG_OFFSET
+                    - sizeof(X86LegacyXSaveArea)
+                    - sizeof(X86XSaveHeader)
+                    - sizeof(XSaveAVX)];
+
     /* MPX State: */
     XSaveBNDREG bndreg_state;
     XSaveBNDCSR bndcsr_state;