Pull lxcContainerGetSubtree out into shared virfile module
    
    Move the code for lxcContainerGetSubtree into the virfile
    module creating 2 new functions
    
      int virFileGetMountSubtree(const char *mtabpath,
                                 const char *prefix,
                                 char ***mountsret,
                                 size_t *nmountsret);
      int virFileGetMountReverseSubtree(const char *mtabpath,
                                        const char *prefix,
                                        char ***mountsret,
                                        size_t *nmountsret);
    
    Add a new virfiletest.c test case to validate the new code.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>