Introduce support for domainGetJobInfo to get info about the
ongoing job. If the job is active it will update the
timeElapsed which is computed with the "started" field added to
struct libxlDomainJobObj.  For now we support just the very basic
info and all jobs have VIR_DOMAIN_JOB_UNBOUNDED (i.e. no completion
time estimation) plus timeElapsed computed.

Openstack Kilo uses the Job API to monitor live-migration
progress which is currently nonexistent in libxl driver and
therefore leads to a crash in the nova compute node. Right
now, migration doesn't use jobs in the source node and will
return VIR_DOMAIN_JOB_NONE. Though nova handles this case and
will migrate it properly instead of crashing.
Signed-off-by: NJoao Martins <joao.m.martins@oracle.com>