webadmin: Validate VM disk aliases when creating template

When creating a template from a VM, validate the future disk aliases of the
template so they are not empty and contain only alphanumeric chars

Change-Id: I9da39e89c6ae2f289363983e235e65f1d09687ec
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1110309