target/ppc: Migrate DECR SPR

TCG does not maintain the DEC reigster in the SPR array, so it does get
migrated. TCG also needs to re-start the decrementer timer on the
destination machine.

Load and store the decrementer into the SPR when migrating. This works
for the level-triggered (book3s) decrementer, and should be compatible
with existing KVM machines that do keep the DEC value there.

This fixes lost decrementer interrupt on migration that can cause
hangs, as well as other problems including record-replay bugs.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>