Xen 4.5.x and earlier enables certain default backends when emulating a VGA device for an x86 HVM guest qemu even when the configuration disables them, which allows local guest users to obtain access to the VGA console by (1) setting the DISPLAY environment variable, when compiled with SDL support, or connecting to the VNC server on (2) ::1 or (3) 127.0.0.1, when not compiled with SDL support.
https://security.gentoo.org/glsa/201504-04
http://xenbits.xen.org/xsa/advisory-119.html
http://www.securitytracker.com/id/1031919
http://www.securitytracker.com/id/1031806
http://www.securityfocus.com/bid/73068
http://lists.opensuse.org/opensuse-security-announce/2015-04/msg00014.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-March/152776.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-March/152588.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-March/152483.html