Moving data reception of the perform phase of migration to a
thread introduces a race with the finish phase, where checking
if the domain is active races with the thread finishing the
perform phase.  The race is easily solved by acquiring a job in
the finish phase, which must wait for the perform phase job to
complete.

While wrapping the finish phase in a job, noticed the virDomainObj
was being unlocked in a callee - libxlDomainMigrationFinish.  Move
the unlocking to libxlDomainMigrateFinish3Params, where the lock
is acquired.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>