Disable revert to / remove snapshot button if no snapshots exist

https://bugzilla.redhat.com/show_bug.cgi?id=1096746
Update specs for ApplicationHelper

build_toolbar_disable_button() method now returns (error) message
in case no snapshots for VM exist. The routine now returns false
only in case the VM does have snapshots.

https://bugzilla.redhat.com/show_bug.cgi?id=1096746
Merge pull request #769 from mzazrivec/fix_disable_snapshot_button

Disable revert to / remove snapshot button if no snapshots exist