vhost-user-blk: add immediate cleanup on shutdown

vhost-user-blk: add immediate cleanup on shutdown

Qemu crashes on shutdown if the chardev used by vhost-user-blk has been
finalized before the vhost-user-blk.

This happens with char-socket chardev operating in the listening mode (server).
The char-socket chardev emits "close" event at the end of finalizing when
its internal data is destroyed. This calls vhost-user-blk event handler
which in turn tries to manipulate with destroyed chardev by setting an empty
event handler for vhost-user-blk cleanup postponing.

This patch separates the shutdown case from the cleanup postponing removing
the need to set an event handler.

Signed-off-by: Denis Plotnikov <den-plotnikov@yandex-team.ru>
Message-Id: <20210325151217.262793-4-den-plotnikov@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>