qemu: Refactor qemuMigrationUpdateJobStatus

qemu: Refactor qemuMigrationUpdateJobStatus

Once we start waiting for migration events instead of polling
query-migrate, priv->job.current will not be regularly updated anymore
because we will get the current status directly from the events. Thus
virDomainGetJob{Info,Stats} will have to query QEMU, but they can't just
blindly update priv->job.current structure. This patch introduces
qemuMigrationFetchJobStatus which just fills in a caller supplied
structure and makes qemuMigrationUpdateJobStatus a tiny wrapper around
it.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>