Description of problem:
Scenario: you want to migrate Hosted Engine VM from within the webadmin gui.
It would be nice to pre-filter the proposed drop down box when you have to choose the target host where migrate the hosted engine...
So that if there are no hosts available you will be given a related message without choice at all and if there is a subset of eligible hosts inside the cluster (because not all are configured as Hosted Engine enabled in general), you will be proposed only to choose one of them and not all the hosts inside the cluster.

Version-Release number of selected component (if applicable):
4.0.5

How reproducible:
always

Steps to Reproduce:
1. 3 hosts with only host1 and host2 configured as Hosted Engine.
Hosted engine running on host1.
2. from the gui select HE and "migrate"; don't choose the default option "select host automatically", but choose "select destination host" option: you are proposed host2 and host3. 
3. Choose host3, that is NOT configured as hosted engine

Actual results:
You get this error
https://drive.google.com/file/d/0BwoPbcrMv8mvY3pURVRkX0p4OW8/view?usp=sharing

Expected results:
The drop down list could be pre-filtered so that one cannot choose a host that is already known would give an error message.

Additional info:
In case of no further hosts enabled for Hosted Engine, apart where is already running, the user will get a window telling that he/she cannot migrate the HE VM and that it is advisable to add more hosts configured as Hosted Engine to guarantee HA