Security update for xen (important)

This update for xen fixes several issues.

These security issues were fixed:

• A malicious 64-bit PV guest may be able to access all of system memory,
  allowing for all of privilege escalation, host crashes, and information
  leaks by placing a IRET hypercall in the middle of a multicall batch
  (XSA-213, bsc#1034843)
• A malicious pair of guests may be able to access all of system memory,
  allowing for all of privilege escalation, host crashes, and information
  leaks because of a missing check when transfering pages via
  GNTTABOP_transfer (XSA-214, bsc#1034844).
• CVE-2017-7718: hw/display/cirrus_vga_rop.h allowed local guest OS
  privileged users to cause a denial of service (out-of-bounds read and
  QEMU process crash) via vectors related to copying VGA data via the
  cirrus_bitblt_rop_fwd_transp_ and cirrus_bitblt_rop_fwd_ functions
  (bsc#1034994).
• CVE-2016-9603: A privileged user within the guest VM could have caused a
  heap overflow in the device model process, potentially escalating their
  privileges to that of the device model process (bsc#1028655)

These non-security issues were fixed:

• bsc#1027519: Missing upstream bug fixes
• bsc#1015348: libvirtd does not start during boot
• bsc#1022555: Timeout in "execution of /etc/xen/scripts/block add