The Xen documentation in install.rst was out of date, describing
xm-specific changes at the point where 2.12 is mostly used with xl.
This patch removes xm-specific migration steps, references the official
Xen wiki instead of replicating information from it, removes the
VNC setup settings that are outdated for xl and probably for xm, and
slightly rewrites the documentation.

Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>