virt-login-shell: add ability to auto-detect shell from container
    
    Currently the shell must be looked up from the config setting in
    /etc/libvirt/virt-login-shell.conf. This is inflexible if there
    are containers where different users need different shells. Add
    add a new 'auto-shell' config parameter which instructs us to
    query the containers' /etc/passwd for the shell to be exec'd.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>