core: prevent performing preview when the vm has illegal disks

When attempting to preview vm - do not allow to perform it when the vm
has illegal disks contained within the filtered disk list.

Change-Id: I8ee0e8070fb91bd2e0f18cf7bdb7a44aa51f70af
Bug-Url: https://bugzilla.redhat.com/949624
Signed-off-by: Liron Aravot <laravot@redhat.com>