qemu: Make updating stats in qemuMigrationCheckJobStatus optional

qemu: Make updating stats in qemuMigrationCheckJobStatus optional

With new QEMU which supports migration events,
qemuMigrationCheckJobStatus needs to explicitly query QEMU for migration
statistics once migration is completed to make sure the caller sees
up-to-date statistics with both old and new QEMU. However, some callers
are not interested in the statistics at all and once we start waiting
for a completed migration on the destination host too, checking the
statistics would even fail. Let's push the decision whether to update
the statistics or not to the caller.

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