qemuBackupBegin: Fix monitor access when rolling back due to failure

The code attempting to clean up after a failed pull mode backup job
wrongly entered monitor but didn't clean up nor exit monitor due to a
logic bug. Fix the condition.

Introduced in a1521f84

https://bugzilla.redhat.com/show_bug.cgi?id=1817327Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

src/qemu/qemu_backup.c

@@ -894,7 +894,7 @@ qemuBackupBegin(virDomainObjPtr vm,
     qemuCheckpointRollbackMetadata(vm, chk);
 
     if (!job_started && nbd_running &&
-        qemuDomainObjEnterMonitorAsync(priv->driver, vm, QEMU_ASYNC_JOB_BACKUP) < 0) {
+        qemuDomainObjEnterMonitorAsync(priv->driver, vm, QEMU_ASYNC_JOB_BACKUP) == 0) {
         ignore_value(qemuMonitorNBDServerStop(priv->mon));
         ignore_value(qemuDomainObjExitMonitor(priv->driver, vm));
     }