qemu_ram_unset_idstr() doesn't need rcu lock anymore,
meanwhile make the range of rcu lock in
qemu_ram_set_idstr() as small as possible.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Message-Id: <1462845901-89716-3-git-send-email-arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>