qemu_migrate: use nested job when adding NBD to cookie

qemuMigrationCookieAddNBD is usually called from within an async
MIGRATION_OUT or MIGRATION_IN job, so it needs to start a nested job.

(The one exception is during the Begin phase when change protection
isn't enabled, but qemuDomainObjEnterMonitorAsync will behave the same
as qemuDomainObjEnterMonitor in this case.)

This bug was encountered with a libvirt client that repeatedly queries
the disk mirroring block job info during a migration. If one of these
queries occurs just as the Perform migration cookie is baked, libvirt
crashes.

Relevant logs are as follows:

    6701: warning : qemuDomainObjEnterMonitorInternal:1544 : This thread seems to be the async job owner; entering monitor without asking for a nested job is dangerous
[1] 6701: info : qemuMonitorSend:972 : QEMU_MONITOR_SEND_MSG: mon=0x7fefdc004700 msg={"execute":"query-block","id":"libvirt-629"}
[2] 6699: info : qemuMonitorIOWrite:503 : QEMU_MONITOR_IO_WRITE: mon=0x7fefdc004700 buf={"execute":"query-block","id":"libvirt-629"}
[3] 6704: info : qemuMonitorSend:972 : QEMU_MONITOR_SEND_MSG: mon=0x7fefdc004700 msg={"execute":"query-block-jobs","id":"libvirt-630"}
[4] 6699: info : qemuMonitorJSONIOProcessLine:203 : QEMU_MONITOR_RECV_REPLY: mon=0x7fefdc004700 reply={"return": [...], "id": "libvirt-629"}
    6699: error : qemuMonitorJSONIOProcessLine:211 : internal error: Unexpected JSON reply '{"return": [...], "id": "libvirt-629"}'

At [1] qemuMonitorBlockStatsUpdateCapacity sends its request, then waits
on mon->notify. At [2] the request is written out to the monitor socket.
At [3] qemuMonitorBlockJobInfo sends its request, and also waits on
mon->notify. The reply from the first request is received at [4].
However, qemuMonitorJSONIOProcessLine is not expecting this reply since
the second request hadn't completed sending. The reply is dropped and an
error is returned.

qemuMonitorIO signals mon->notify twice during its error handling,
waking up both of the threads waiting on it. One of them clears mon->msg
as it exits qemuMonitorSend; the other crashes:

  qemuMonitorSend (mon=0x7fefdc004700, msg=<value optimized out>) at qemu/qemu_monitor.c:975
  975         while (!mon->msg->finished) {
  (gdb) print mon->msg
  $1 = (qemuMonitorMessagePtr) 0x0
Signed-off-by: NMichael Chapman <mike@very.puzzling.org>

src/qemu/qemu_migration.c

@@ -571,7 +571,9 @@ qemuMigrationCookieAddNBD(qemuMigrationCookiePtr mig,
             if (!(stats = virHashCreate(10, virHashValueFree)))
                 goto cleanup;
 
-            qemuDomainObjEnterMonitor(driver, vm);
+            if (qemuDomainObjEnterMonitorAsync(driver, vm,
+                                               priv->job.asyncJob) < 0)
+                goto cleanup;
             rc = qemuMonitorBlockStatsUpdateCapacity(priv->mon, stats, false);
             if (qemuDomainObjExitMonitor(driver, vm) < 0)
                 goto cleanup;