virschematest: Access the right directory containing XMLs

virschematest: Access the right directory containing XMLs

So the story goes like this. The testSchemaDirs() function is
called with: a) the schema file, b) list of the directories that
contains XMLs documents that should be checked against the schema
file from a). However, the directories in the list are really
just their names and it's up to testSchemaDirs to construct the
absolute path and call testSchemaDir() which then does the actual
validation. The absolute path is constructed, but never actually
used (maybe due to a typo). Thus a VPATH build is broken.

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