Security update for xen (important)

This update for xen fixes the following issues:

• A Mishandling of SYSCALL singlestep during emulation which could have
  lead to privilege escalation. (XSA-204, bsc#1016340, CVE-2016-10013)
• CMPXCHG8B emulation failed to ignore operand size override which could
  have lead to information disclosure. (XSA-200, bsc#1012651,
  CVE-2016-9932)
• PV guests may have been able to mask interrupts causing a Denial of
  Service. (XSA-202, bsc#1014298, CVE-2016-10024)
• A missing NULL pointer check in VMFUNC emulation could lead to a
  hypervisor crash leading to a Denial of Servce. (XSA-203, bsc#1014300,
  CVE-2016-10025)