Univention Corporate Server 4.2 erratum 415 (security)

This update addresses the following issues:
* Null pointer dereference when updating storage size on empty drives
  (CVE-2017-2635)
* libvirt is vulnerable to a bad default configuration of "verify-peer=no"
  passed to QEMU by libvirt resulting in a failure to validate SSL/TLS
  certificates by default (CVE-2017-1000256)
* Incomplete fix for CVE-2018-5748 triggered by QEMU guest agent
  (CVE-2018-1064)
* guest could inject executable code via libnss_dns.so loaded by libvirt_lxc
  before init (CVE-2018-6764)