lib: Replace virBuildPath() with g_build_filename()

lib: Replace virBuildPath() with g_build_filename()

Our virBuildPath() constructs a path from given arguments.
Exactly like g_build_filename(), except the latter is more
generic as it uses backslashes on Windows. Therefore, replace the
former with the latter.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>