With that users could access files outside /dev/shm.  That itself
isn't a security problem, but might cause some errors we want to
avoid.  So let's forbid slashes as we do with domain and volume names
and also mention that in the schema.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1395496Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>