description of problem:
libvirt should report error when revert to a internal snapshot when we are in external snapshot

Version-Release number of selected component (if applicable):
libvirt-1.2.8-10.el7.x86_64
qemu-kvm-rhev-2.1.2-17.el7.x86_64

How reproducible:
100%

Steps to Reproduce:

1.prepare a running guest have a qcow2 disk
# virsh dumpxml test3
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/var/lib/libvirt/images/test6.img'/>
      <target dev='hda' bus='ide'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

2.do internal snapshot
# virsh snapshot-create-as test3 s1

Domain snapshot s1 created

3.edit some thing in guest

4.create a external snapshot:
# virsh snapshot-create-as test3 s2 --disk-only
Domain snapshot s2 created

5.revert to internal snapshot s1:
# virsh snapshot-revert test3 s1

# echo $?
0


Actual results:
no error when do snapshot-revert and nothing changed after revert(even reboot then check)
      
Expected results:
output error when snapshot-revert failed, like this:

the snapshot 's1' does not exist, and was not loaded


Some Information:

After i debug libvirt and qemu, i found :
libvirt:                                qemu-kvm:
qemuDomainRevertToSnapshot
    qemuMonitorTextLoadSnapshot
        qemuMonitorHMPCommand
                                        do_loadvm
                                             load_vmstate
                                                 bdrv_snapshot_find
                                               /*cannot find a snapshot*/
                                             return ret
                                        end     
                                        /*no error output when cannot find */
        /*reply = '' so no error */
        ret = 0

And libvirt try to find a error like this:

    else if (strstr(reply, "Could not find snapshot") != NULL) {
        virReportError(VIR_ERR_OPERATION_INVALID,
                       _("the snapshot '%s' does not exist, and was not loaded"),
                       name);
        goto cleanup;
    }

But with qemu-kvm-rhev seems qemu will never output this reply.
So libvirt return success.