qemu: Don't unref domain after exit from nested async job

In commit 540c339a the whole domain
reference counting was refactored in the qemu driver. Domain jobs now
don't need to reference the domain object as they now expect the
reference from the calling function.

However, the patch forgot to remove the unref call in case we exit the
monitor when we were acquiring a nested job. This caused the daemon to
crash on a subsequent access to the domain object once we've done an
operation requiring a nested job for a monitor access.

An easy reproducer case:

1) Start a vm with qcow disks
2) virsh snapshot-create-as DOMNAME
3) virsh dumpxml DOMNAME
4) daemon crashes in a semi-random spot while accessing a now-removed VM
object.

Fortunately, the commit wasn't released yet, so there are no security
implications.
Reported-by: NShanzi Yu <shyu@redhat.com>
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

src/qemu/qemu_domain.c

@@ -1573,8 +1573,6 @@ qemuDomainObjExitMonitorInternal(virQEMUDriverPtr driver,
         qemuDomainObjResetJob(priv);
         qemuDomainObjSaveJob(driver, obj);
         virCondSignal(&priv->job.cond);
-
-        virObjectUnref(obj);
     }
 }