SUSE-SU-2020:14557-1 Security update for xen

This update for xen fixes the following issues: - bsc1178963 - stack corruption from XSA-346 change XSA-355 - bsc1178935 - CVE-2020-25723: assertion failure through usbpacketunmap in hw/usb/hcd-ehci.c - bsc1177409 - CVE-2020-27674: x86 PV guest INVLPG-like flushes may leave stale TLB entries...