tests: Fix virQEMUDriverConfigNew() calling with respect to @root

tests: Fix virQEMUDriverConfigNew() calling with respect to @root

The virQEMUDriverConfigNew() accepts path to root directory for
embed mode as an argument. If the argument is not NULL it uses
the passed value as prefix for some internal paths (e.g.
cfg->libDir). If it is NULL though, it looks if the other
argument, @privileged is true or false and generates internal
paths accordingly. But when calling the function from the test
suite, instead of passing NULL for @root, an empty string is
passed. Fortunately, this doesn't create a problem because in
both problematic cases the generated paths are "fixed" to point
somewhere into build dir or the code which is tested doesn't
access them.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>