This change adds a DELETE call on the server-migrations object to cancel
a running live migration of a specific instance.
TO perform the cancellation the virtualization driver needs to support
it, in case that the feature is not supported we return an error.
We allow a cancellation of a migration only if the migration is
running at the moment of the request and if the migration type is equal
to 'live-migration'.
In this change we implement this feature for the libvirt driver.
When the cancellation of a live migration succeeded we rollback the live
migration and we set the state of the Migration object equals to
'cancelled'.
The implementation of this change is based on the work done by the
implementation of the feature called 'force live migration':
https://review.openstack.org/245921
DocImpact
ApiImpact
Implements blueprint: abort-live-migration
Change-Id: I1ff861e54997a069894b542bd764ac3ef1b3dbb2
fa00292546