migration: move vm_old_running into global state

Firstly, it was passed around.  Let's just move it into MigrationState
just like many other variables as state of migration, renaming it to
vm_was_running.

One thing to mention is that for postcopy, we actually don't need this
knowledge at all since postcopy can't resume a VM even if it fails (we
can see that from the old code too: when we try to resume we also check
against "entered_postcopy" variable).  So further we do this:

- in postcopy_start(), we don't update vm_old_running since useless
- in migration_thread(), we don't need to check entered_postcopy when
  resume, since it's only used for precopy.

Comment this out too for that variable definition.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
