The remote host is missing an update to xen announced via advisory DSA 2544-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%202544-1

Multiple denial of service vulnerabilities have been discovered in xen, an hypervisor. The Common Vulnerabilities and Exposures project identifies the following problems: CVE-2012-3494: It was discovered that set_debugreg allows writes to reserved bits of the DR7 debug control register on amd64 (x86-64) paravirtualised guests, allowing a guest to crash the host. CVE-2012-3496: Matthew Daley discovered that XENMEM_populate_physmap, when called with the MEMF_populate_on_demand flag set, a BUG (detection routine) can be triggered if a translating paging mode is not being used, allowing a guest to crash the host. For the stable distribution (squeeze), these problems have been fixed in version 4.0.1-5.4. For the testing distribution (wheezy), these problems will be fixed soon. For the unstable distribution (sid), these problems have been fixed in version 4.1.3-2. We recommend that you upgrade your xen packages.