debian: split NEWS file into package specific ones

debian: split NEWS file into package specific ones

Users who only have some libxen* package installed do not have to be
presented with scary news items that are relevant only if you have
hypervisor and/or tools installed.

Besides that, the messages are mostly really package specific. And,
there's no strict enforcement (because the user is able to have multiple
xen versions installed) to have exact the same version for all xen
packages. If you install a newer xen-hypervisor-common, you won't get
the init script fixes for example that apt-listchanges would be blaring
about.

(Closes: #962267)

Signed-off-by: Hans van Kranenburg <hans@knorrie.org>