KVM: Rework VCPU state writeback API

This grand cleanup drops all reset and vmsave/load related
synchronization points in favor of four(!) generic hooks:

- cpu_synchronize_all_states in qemu_savevm_state_complete
  (initial sync from kernel before vmsave)
- cpu_synchronize_all_post_init in qemu_loadvm_state
  (writeback after vmload)
- cpu_synchronize_all_post_init in main after machine init
- cpu_synchronize_all_post_reset in qemu_system_reset
  (writeback after system reset)

These writeback points + the existing one of VCPU exec after
cpu_synchronize_state map on three levels of writeback:

- KVM_PUT_RUNTIME_STATE (during runtime, other VCPUs continue to run)
- KVM_PUT_RESET_STATE   (on synchronous system reset, all VCPUs stopped)
- KVM_PUT_FULL_STATE    (on init or vmload, all VCPUs stopped as well)

This level is passed to the arch-specific VCPU state writing function
that will decide which concrete substates need to be written. That way,
no writer of load, save or reset functions that interact with in-kernel
KVM states will ever have to worry about synchronization again. That
also means that a lot of reasons for races, segfaults and deadlocks are
eliminated.

cpu_synchronize_state remains untouched, just as Anthony suggested. We
continue to need it before reading or writing of VCPU states that are
also tracked by in-kernel KVM subsystems.

Consequently, this patch removes many cpu_synchronize_state calls that
are now redundant, just like remaining explicit register syncs.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

target-i386/machine.c

@@ -321,8 +321,6 @@ static void cpu_pre_save(void *opaque)
     CPUState *env = opaque;
     int i;
 
-    cpu_synchronize_state(env);
-
     /* FPU */
     env->fpus_vmstate = (env->fpus & ~0x3800) | (env->fpstt & 0x7) << 11;
     env->fptag_vmstate = 0;
@@ -337,14 +335,6 @@ static void cpu_pre_save(void *opaque)
 #endif
 }
 
-static int cpu_pre_load(void *opaque)
-{
-    CPUState *env = opaque;
-
-    cpu_synchronize_state(env);
-    return 0;
-}
-
 static int cpu_post_load(void *opaque, int version_id)
 {
     CPUState *env = opaque;
@@ -373,7 +363,6 @@ static const VMStateDescription vmstate_cpu = {
     .minimum_version_id = 3,
     .minimum_version_id_old = 3,
     .pre_save = cpu_pre_save,
-    .pre_load = cpu_pre_load,
     .post_load = cpu_post_load,
     .fields      = (VMStateField []) {
         VMSTATE_UINTTL_ARRAY(regs, CPUState, CPU_NB_REGS),