Add User#accessible_vms

to return all the VMs that are accessible to the user by his role.

https://bugzilla.redhat.com/show_bug.cgi?id=1138234
Add test cases for User#accessible_vms.

https://bugzilla.redhat.com/show_bug.cgi?id=1138234
Check the user's access right when creating a chargeback report by tag.

https://bugzilla.redhat.com/show_bug.cgi?id=1138234