libxl: implement virDomainGetJobStats

libxl: implement virDomainGetJobStats

Introduces support for domainGetJobStats which has the same
info as domainGetJobInfo but in a slightly different format.
Another difference is that virDomainGetJobStats can also
retrieve info on the most recently completed job. Though so
far this is only used in the source node to know if the
migration has been completed. But because we don't support
completed jobs we will deliver an error.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>