kvm-all: Sync dirty-bitmap from kvm before kvm destroy the corresponding dirty_bitmap

kvm-all: Sync dirty-bitmap from kvm before kvm destroy the corresponding dirty_bitmap

Sometimes, we destroy the dirty_bitmap in kvm_memory_slot before any sync action
occur, this bit in dirty_bitmap will be missed, and which will lead the corresponding
dirty pages to be missed in migration.

This usually happens when do migration during VM's Start-up or Reboot.

Signed-off-by: zhanghailiang <[email protected]>
[Use s->migration_log instead of exec.c's in_migration. - Paolo]
Signed-off-by: Paolo Bonzini <[email protected]>