Convert the stats field in _qemuDomainJobInfo to be a union. This
will allow for the collection of various different types of stats
in the same field.

When starting the async job that will end up being used for stats,
set the @statsType value appropriately. The @mirrorStats are
special and are used with stats.mig in order to generate the
returned job stats for a migration.

Using the NONE should avoid the possibility that some random
async job would try to return stats for migration even though
a migration is not in progress.

For now a migration and a save job will use the same statsType
Reviewed-by: NJiri Denemark <jdenemar@redhat.com>