libxl: MigratePrepare: use standard begin and end API pattern

libxl: MigratePrepare: use standard begin and end API pattern

libxlDomainMigrationPrepare adds the incoming domain def to the list
of domains via virDomainObjListAdd, but never adds its own ref to the
returned virDomainObj as other callers of virDomainObjListAdd do.
libxlDomainMigrationPrepareTunnel3 suffers the same discrepancy.

Change both to add a ref to the virDomainObj after a successful
virDomainObjListAdd, similar to other callers. This ensures a consistent
pattern throughout the drivers and allows using the virDomainObjEndAPI
function for cleanup.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>