qemu: Don't try to fetch migration stats on destination

qemu: Don't try to fetch migration stats on destination

Migration statistics are not available on the destination host and
starting a query job during incoming migration is not allowed. Trying to
do that would result in

    Timed out during operation: cannot acquire state change lock (held
    by remoteDispatchDomainMigratePrepare3Params)

error. We should not even try to start the job.

https://bugzilla.redhat.com/show_bug.cgi?id=1278727

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