libxl: vncviewer: make autopass work properly

libxl: vncviewer: make autopass work properly

The file we write the vnc password to must be rewound back to the
beginning, or the vnc viewer will simply get EOF.

When the syscalls for communicating the password to the vnc client
fail, bomb out with an error messsage rather than blundering on (and
probably producing a spurious password prompt).

Following this patch, xl vncviewer --autopass works, provided the qemu
patch for writing the password to xenstore has also been applied.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>