Introduce bash completion
    
    With this change one can get list of domains on the command line:
    
      $ virt-viewer -c qemu:///system <TAB><TAB>
      dom1   dom2   ... domN
    
    The list of domains is fetched using virsh, hence the dependency
    on libvirt-client recorded in the spec file. I think it's fair
    to assume that Linux hosts with virt-viewer will have virsh
    available too. If they don't, nothing breaks and no error message
    is printed.
    
    The completer script is inspired by libvirt.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>