Import the Xend parts of xen-unstable changeset

Import the Xend parts of xen-unstable changeset
11840:02506a7443155611d6bbf03e49fbf193e96d24db.

[HVM] Implement password authentication of VNC connections.

The specification is as mentioned at
http://lists.xensource.com/archives/html/xen-devel/2006-09/msg00666.html
(However, password came to describe plain text)

The difference is follows.
- protocol_authtype() without the necessity was deleted.
- The check on the protocol version was added.
- And, some small modification.

Signed-off-by: Masami Watanabe <masami.watanabe@jp.fujitsu.com>