migration: Add 'pre-switchover' and 'device' statuses

migration: Add 'pre-switchover' and 'device' statuses

Add two statuses for use when the 'pause-before-switchover'
capability is enabled.

'pre-switchover' is the state that we wait in for management
to allow us to continue.
'device' is the state we enter while serialising the devices
after management gives us the OK.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>