As KVM now makes use of exception_index to keep pending exceptions, we
have to save&restore this field as well.

NOTE: We have to nail the arch-independent exception_index down to a
certain bit width for proper vmstate processing, namely to 32 bit.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>