domblkstat: Produce error message that at least sounds like English
    
    Compare:
    
      # virsh domblkstat freebsd hdd
      error: Failed to get block stats freebsd hdd
      error: invalid argument: invalid path: hdd
    
    with:
    
      # virsh domblkstat freebsd hdd
      error: Failed to get block stats for domain 'freebsd' device 'hdd'
      error: invalid argument: invalid path: hdd
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>