fence_scsi: fix registration handling in device 'off' workflows

ISID (Initiator Session ID) belonging to I_T Nexus changes for
RHEL based on the session ID. This means that the connection to
the device can be set up with different ISID on reconnects.

When a device is powered off, fence_scsi assumes that the client
has a registration to the device and sends a preempt-and-abort
request which ends up failing due to reservation conflict.

Fixing this by registering the host key with the device and preempting
the old registration (if it exists). This should make sure that the
host is able to preempt the other key successfully.