During incoming migration the QEMU monitor is not able to be
used. The incoming migration code did not keep hold of the
job lock because migration is split across multiple API calls.
This meant that further monitor commands on the guest would
hang until migration finished with no timeout.

In this change the qemuDomainMigratePrepare method sets the
job flag just before it returns. The qemuDomainMigrateFinish
method checks for this job flag & clears it once done. This
prevents any use of the monitor between prepare+finish steps.

The qemuDomainGetJobInfo method is also updated to refresh
the job elapsed time. This means that virsh domjobinfo can
return time data during incoming migration

* src/qemu/qemu_driver.c: Keep a job active during incoming
  migration. Refresh job elapsed time when returning job info