vhost-user: add ability to know vhost-user backend disconnection

Current QEMU cannot detect vhost-user backend disconnection. The
patch adds ability to know it.
To know disconnection, add watcher to detect G_IO_HUP event. When
G_IO_HUP event is detected, the disconnected socket will be read
to cause a CHR_EVENT_CLOSED.

Signed-off-by: Tetsuya Mukawa <mukawa@igel.co.jp>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Victor Kaplansky <victork@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>