target-arm: kvm64 fix save/restore of SPSR regs

The current code was negatively indexing the cpu state array and not
synchronizing banked spsr register state with the current mode's spsr
state, causing occasional failures with migration.

Some munging is done to take care of the aarch64 mapping and also to
ensure the most current value of the spsr is updated to the banked
registers (relevant for KVM<->TCG migration).

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
