webadmin: Fix vm pop-up host tab logic

The logic of the host tab in the VM pop-up view was inconsistent.

The EntityModel of the UnitVmModel migration support was changed to
match the other models.

UI was changed - instead of checkboxs which mutually exclude each other,
a new list-box was added to select the migration mode.

Change-Id: Ib4da1be4919913b7739f4139299d2c88958488d0
Bug-Url: https://bugzilla.redhat.com/905831
Signed-off-by: Ofri Masad <omasad@redhat.com>