tcm_vhost: Refactor the lock nesting rule

We want to use tcm_vhost_mutex to make sure hotplug/hotunplug will not
happen when set_endpoint/clear_endpoint is in process.

Signed-off-by: Asias He <asias@redhat.com>
tcm_vhost: Add hotplug/hotunplug support

In commit 365a7150094 ([SCSI] virtio-scsi: hotplug support for
virtio-scsi), hotplug support is added to virtio-scsi.

This patch adds hotplug and hotunplug support to tcm_vhost.

You can create or delete a LUN in targetcli to hotplug or hotunplug a
LUN in guest.

Changes in v8:
- Use vq->mutex for event
- Drop tcm_vhost: Add helper to check if endpoint is setup
- Rename vs_events_dropped to vs_events_missed
- Init lun[] explicitly

Changes in v7:
- Add vhost_work_flush for vs->vs_event_work to this series

Changes in v6:
- Pass tcm_vhost_evt to tcm_vhost_do_evt_work

Changes in v5:
- Switch to int from u64 to vs_events_nr
- Set s->vs_events_dropped flag in tcm_vhost_allocate_evt
- Do not nest dev mutex within vq mutex
- Use vs_events_lock to protect vs_events_dropped and vs_events_nr
- Rebase to target/master

Changes in v4:
- Drop tcm_vhost_check_endpoint in tcm_vhost_send_evt
- Add tcm_vhost_check_endpoint in vhost_scsi_evt_handle_kick

Changes in v3:
- Separate the bug fix to another thread

Changes in v2:
- Remove code duplication in tcm_vhost_{hotplug,hotunplug}
- Fix racing of vs_events_nr
- Add flush fix patch to this series

Signed-off-by: Asias He <asias@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
tcm_vhost: Add ioctl to get and set events missed flag

Signed-off-by: Asias He <asias@redhat.com>