When the AHCI HBA device is migrated, all of the information that
led to the request being created is stored in the AHCIDevice
structures, except for pointers into guest data where return
information needs to be stored.

The "cur_cmd" field is usually responsible for this.

To rebuild the cur_cmd pointer post-migration, we can utilize
the busy_slot index to figure out where the command header
we are still processing is.

This allows a machine in a halted state from rerror=stop or
werror=stop to be migrated and resume operations without issue.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Message-id: 1424708286-16483-17-git-send-email-jsnow@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>