xen: Check return value of virStringReplace
    
    After 6604a3dd9f8 in which new helper function has been
    introduced, the code calls virStringReplace and dereference the
    result immediately. The string function can, however, return NULL
    so this would SIGSEGV right away. Check for the return value of
    the string function.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>