xen - security update

Multiple security issues have been found in the Xen virtualisation
solution, which may result in denial of service or information disclosure.

The oldstable distribution (wheezy) will be updated in a separate DSA.

For the stable distribution (jessie), these problems have been fixed in
version 4.4.1-9+deb8u4.

For the unstable distribution (sid), these problems will be fixed soon.

We recommend that you upgrade your xen packages.