schema: Don't validate paths

schema: Don't validate paths

https://bugzilla.redhat.com/show_bug.cgi?id=1353296

On UNIX like systems there are no constraints on what characters
can be in file/dir names (except for NULL, obviously). Moreover,
some values that we think of as paths (e.g. disk source) are not
necessarily paths at all. For instance, some hypervisors take
that as an arbitrary identifier and corresponding file is then
looked up by hypervisor in its table. Instead of trying to fix
our regular expressions (and forgetting to include yet another
character there), lets drop the validation completely.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>