xen-api: Change PAM configuration from password-auth to passwd

password-auth isn't available on Ubuntu, but passwd is and it
seems to behave equivalently to password-auth on CentOS.

In particular, local non-root users can still call xe commands using:

  xe -u root -pw <password> <command>

Signed-off-by: Euan Harris <euan.harris@citrix.com>