libvirt does not logout of iscsi targets, causing system hang on shutdown

libvirt does not logout of iscsi targets, causing system hang on shutdown

There's a quite old bug entry here:

https://bugzilla.redhat.com/show_bug.cgi?id=700010

I just stumbled over that very issue on F18. Doing a little bit
debugging of the shutdown sequence, it turns out that - at least on my
F18 installation - libvirtd is shutdown *after* iscsid, which makes it
impossible for libvirt to perform the logout of the iscsi session properly.

This patch simply adds another startup dependancy on iscsid.service
which in turn delays iscsid shutdown until after libvirtd has stopped.
Having that applied, the system shuts down properly again.

Signed-off-by: Eric Blake <eblake@redhat.com>