vhost-user: disconnect on HUP

vhost-user: disconnect on HUP

In some cases, qemu_chr_fe_read_all() on HUP event doesn't raise
CHR_EVENT_CLOSED because the read/recv function returns -1 on
disconnected peers (for example with tch_chr_recv, an ECONNRESET errno
overwritten as EIO).

It is simpler to explicitely disconnect on HUP, rising CHR_EVENT_CLOSED
if it wasn't disconnected already.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>