Security update for xen (important)

This update for xen fixes the following security issues:

• blkif responses leaked backend stack data, which allowed unprivileged
  guest to obtain sensitive information from the host or other guests
  (XSA-216, bsc#1042863)
• Page transfer might have allowed PV guest to elevate privilege (XSA-217,
  bsc#1042882)
• Races in the grant table unmap code allowed for informations leaks and
  potentially privilege escalation (XSA-218, bsc#1042893)
• Insufficient reference counts during shadow emulation allowed a
  malicious pair of guest to elevate their privileges to the privileges
  that XEN runs under (XSA-219, bsc#1042915)
• Stale P2M mappings due to insufficient error checking allowed malicious
  guest to leak information or elevate privileges (XSA-222, bsc#1042931)
• Grant table operations mishandled reference counts allowing malicious
  guests to escape (XSA-224, bsc#1042938)
• CVE-2017-9330: USB OHCI Emulation in qemu allowed local guest OS users
  to cause a denial of service (infinite loop) by leveraging an incorrect
  return value (bsc#1042160)
• CVE-2017-8309: Memory leak in the audio/audio.c allowed remote attackers
  to cause a denial of service (memory consumption) by repeatedly starting
  and stopping audio capture (bsc#1037243)
• CVE-2017-8905: Xen a failsafe callback, which might have allowed PV
  guest OS users to execute arbitrary code on the host OS (XSA-215,
  bsc#1034845).