qemu: Fix return value of qemuDomainGetBlockJobInfo

qemu: Fix return value of qemuDomainGetBlockJobInfo

While reviewing 1b43885d1784640 I've noticed a virReportError()
followed by a goto endjob; without setting the correct return
value. Problem is, if block job is so fast that it's bandwidth
does not fit into ulong, an error is reported. However, by that
time @ret is already set to 1 which means success. Since the
scenario can be hardly considered successful, we should return a
value meaning error.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>