migration: optimize the downtime

Qemu_savevm_state_cleanup takes about 300ms in my ram migration tests
with a 8U24G vm(20G is really occupied), the main cost comes from
KVM_SET_USER_MEMORY_REGION ioctl when mem.memory_size = 0 in
kvm_set_user_memory_region. In kmod, the main cost is
kvm_zap_obsolete_pages, which traverses the active_mmu_pages list to
zap the unsync sptes.

It can be optimized by delaying memory_global_dirty_log_stop to the next
vm_start.

Changes v2->v3:
 - NULL VMChangeStateHandler if it is deleted and protect the scenario
   of nested invocations of memory_global_dirty_log_start/stop [Paolo]

Changes v1->v2:
 - create a VMChangeStateHandler in memory.c to reduce the coupling [Paolo]

Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
Message-Id: <1501237733-2736-1-git-send-email-jianjay.zhou@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
