qemuProcessHandleMigrationStatus: Update migration status more frequently

qemuProcessHandleMigrationStatus: Update migration status more frequently

After Jirka's migration patches libvirt is listening on migration
events from qemu instead of actively polling on the monitor. There is,
however, a little regression (introduced in 6d2edb6a42d0d41). The
problem is, the current status of migration job is updated in
qemuProcessHandleMigrationStatus if and only if migration job was
started. But eventually every asynchronous job may result in
migration. Therefore, since this job is not strictly a
migration job, internal state was not updated and later checks failed:

  virsh # save fedora22 /tmp/fedora22_ble.save
  error: Failed to save domain fedora22 to /tmp/fedora22_ble.save
  error: operation failed: domain save job: is not active

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