core: fix NPE when mounting run once CD

A payload is valid a device (e.g. CD) that has specParams field set with a
structure containing vmPayload key.
when checking for a vmPayload using isPayload method we should fail fast
if specParams is null, avoid NPE and return false.

Change-Id: I7312102ab6f93122f06ae7c7bd6b1c3158e54ea0
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=893115
Signed-off-by: Roy Golan <rgolan@redhat.com>