libxl: destroy domain in migration finish phase on failure

libxl: destroy domain in migration finish phase on failure

This patch contains three domain cleanup improvements in the migration
finish phase, ensuring a domain is properly disposed when a failure is
detected or the migration is cancelled.

The check for virDomainObjIsActive is moved to libxlDomainMigrationFinish,
where cleanup can occur if migration failed and the domain is inactive.

The 'cleanup' label was missplaced in libxlDomainMigrationFinish, causing
a migrated domain to remain in the event of an error or cancelled migration.

In cleanup, the domain was not removed from the driver's list of domains.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>