Security update for xen (important)

This update for xen fixes the following security issues:

• CVE-2017-7228: Broken check in memory_exchange() permited PV guest
  breakout (bsc#1030442).
• CVE-2017-6414: Memory leak in the vcard_apdu_new function in card_7816.c
  in libcacard allowed local guest OS users to cause a denial of service
  (host memory consumption) via vectors related to allocating a new APDU
  object (bsc#1027570).
• CVE-2017-6505: The ohci_service_ed_list function in hw/usb/hcd-ohci.c
  allowed local guest OS users to cause a denial of service (infinite
  loop) via vectors involving the number of link endpoint list descriptors
  (bsc#1028235).