lsi: Handle removal of selected devices

We must not store references to selected devices as they may be
hot-removed. Instead, look up the device based on its tag right before
using it. If the device disappeared, throw an interrupt and disconnect.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>