Revert "kvm: x86: Save/restore exception_index"

This reverts commit ebbc8a3d8e.

As suggested by Jan Kiszka,

  "It was obsoleted by d1793b836f8f123b961c613de1bb1c0c185c84cc and now
   saves/restores a useless field."

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit acb6685fea)

cpu-defs.h

@@ -179,7 +179,7 @@ typedef struct CPUWatchpoint {
                                                                         \
     /* Core interrupt code */                                           \
     jmp_buf jmp_env;                                                    \
-    int32_t exception_index;                                            \
+    int exception_index;                                                \
                                                                         \
     CPUState *next_cpu; /* next CPU sharing TB cache */                 \
     int cpu_index; /* CPU index (informative) */                        \