migration/vmstate: Remove redundant statement in vmstate_save_state_v()

migration/vmstate: Remove redundant statement in vmstate_save_state_v()

The "ret" has been assigned in all branches. It didn't need to be
 assigned separately.

Clang static code analyzer show warning:
  migration/vmstate.c:365:17: warning: Value stored to 'ret' is never read
                ret = 0;
                ^     ~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>