conf: Require nvdimm path in validate step

conf: Require nvdimm path in validate step

Our code expects that a nvdimm has a path defined always. And the
parser does check for that. Well, not fully - only when parsing
<source/> (which is an optional element). So if the element is
not in the XML then the check is not performed and the assumption
is broken. Verify in the memory def validator that a path was
set.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Han Han <hhan@redhat.com>