Security update for xen

Open Source Vulnerabilities SUSE-SU-2020:2789-1

This update for xen fixes the following issues: - CVE-2020-25604: Fixed a race condition when migrating timers between x86 HVM vCPU-s (bsc#1176343,XSA-336) - CVE-2020-25595: Fixed an issue where PCI passthrough code was reading back hardware registers (bsc#1176344,XSA-337) - CVE-2020-25597: Fixed an issue where a valid event channels may not turn invalid (bsc#1176346,XSA-338) - CVE-2020-25596: Fixed a potential denial of service in x86 pv guest kernel via SYSENTER (bsc#1176345,XSA-339) - CVE-2020-25603: Fixed an issue due to missing barriers when accessing/allocating an event channel (bsc#1176347,XSA-340) - CVE-2020-25600: Fixed out of bounds event channels available to 32-bit x86 domains (bsc#1176348,XSA-342) - CVE-2020-25599: Fixed race conditions with evtchn_reset() (bsc#1176349,XSA-343) - CVE-2020-25601: Fixed an issue due to lack of preemption in evtchn_reset() / evtchn_destroy() (bsc#1176350,XSA-344)