Description of problem:

According to stirabos@ migration of vanilla RHEL6-based (ie. normal installation, not appliance) 3.6 HE VM to appliance-based 4.0 HE VM should be supported.

Trying this action flow, the setup fails with following end output:

~~~
[ INFO  ] Trying to get a fresher copy of vm configuration from the OVF_STORE
[ ERROR ] Unable to get vm.conf from OVF_STORE, falling back to initial vm.conf
          Please install and setup the engine in the VM.
          You may also be interested in installing ovirt-guest-agent-common package in the VM.
         
         
          The VM has been rebooted.
          To continue please install oVirt-Engine in the VM
          (Follow http://www.ovirt.org/Quick_Start_Guide for more info).
         
          Make a selection from the options below:
          (1) Continue setup - oVirt-Engine installation is ready and ovirt-engine service is up
          (2) Abort setup
          (3) Power off and restart the VM
          (4) Destroy VM and abort setup
         
          (1, 2, 3, 4)[1]: 3 
[ INFO  ] Creating VM
[ ERROR ] Failed to execute stage 'Closing up': 'OVEHOSTED_VM/subst'
[ INFO  ] Stage: Clean up
[ INFO  ] Stage: Pre-termination
[ INFO  ] Stage: Termination
[ ERROR ] Hosted Engine upgrade failed: this system is not reliable, please check the issue, fix and try again
          Log file is located at /var/log/ovirt-hosted-engine-setup/ovirt-hosted-engine-setup-20160609120617-h4vd5v.log
~~~

Checking currently running HE VM via vnc console, I see it is stucked in boot because it cannot boot from disk.

Starting HE VM again causes HE VM run with old (original) disk and have 3.6 env.

I would expect following flow:

* setup attaches appliance as another disk to existing HE VM
* restarts HE VM and restores backup
* asks for confirmation that everything is OK
* HE VM is powered off
* when global maintenance is over, HE HA agents would start 


Version-Release number of selected component (if applicable):
ovirt-hosted-engine-ha-1.3.5.7-1.el7ev.noarch
ovirt-hosted-engine-ha-2.0.0-1.el7ev.noarch
ovirt-hosted-engine-setup-1.3.7.1-1.el7ev.noarch
ovirt-hosted-engine-setup-2.0.0-1.el7ev.noarch
rhevm-appliance-20160526.0-1.el7ev.noarch


How reproducible:
100%

Steps to Reproduce:
1. 3.6 HE VM environment - HE VM is (vanilla) RHEL6, not an appliance
2. add 4.0 repos and put the env into global maintenance
3. hosted-engine --upgrade-appliance

Actual results:
- during setup process, HE VM was rebooted and is stucked in booting
  (no bootable  device)
- when the VM is rebooted after the setup failure, it runs with old disk

Expected results:
during setup process HE VM runs with 4.0 appliance disk and guest OS is running

Additional info: