Let's introduce QEMU_DOMAIN_JOB_STATUS_POSTCOPY state for job.current->status
instead of checking job.current->stats.status. The latter can be changed
when fetching migration statistics. Moving state function from the variable
and leave only store function seems more managable.

This patch removes all state checking usage of stats except for
qemuDomainGetJobStatsInternal. This place will be handled separately.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>