vhost: return failure if stop virtqueue failed in vhost_dev_stop

This patch captures the error of vhost_virtqueue_stop() in vhost_dev_stop()
and returns the error upward.

Specifically, if QEMU is disconnected from the vhost backend, some actions
in vhost_dev_stop() will fail, such as sending vhost-user messages to the
backend (GET_VRING_BASE, SET_VRING_ENABLE) and vhost_reset_status.

Considering that both set_vring_enable and vhost_reset_status require setting
the specific virtio feature bit, we can capture vhost_virtqueue_stop()'s
error to indicate that QEMU has lost connection with the backend.

This patch is the pre patch for 'vhost-user: return failure if backend crashes
when live migration', which makes the live migration aware of the loss of
connection with the vhost-user backend and aborts the live migration.

Signed-off-by: Haoqian He <haoqian.he@smartx.com>
Message-Id: <20250416024729.3289157-3-haoqian.he@smartx.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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