- Version bump to 6.1.20 (released April 20 2021 by Oracle)

This is a maintenance release. The following items were fixed and/or added:

VMM: Fixed extremely poor VM performance depending on the timing of various actions (regression in 6.1.0)
VMM: Fixed guest OS hanging under certain circumstances when Hyper-V is present (bug #20141)
VMM: Fixed Guru Meditation error when using a nested hypervisor under certain circumstances (bug #20175)
VMM: Fixed a SMAP related host panic affecting Solaris 11.4 systems with Intel Haswell CPUs or later (bug #16068)
OCI: Add cloud-init support for export to OCI and for OCI instance creation
GUI: Fixed "Delete all files" leaving behind Logs/VBoxUI.log (bug #20235)
Audio: Multiple fixes and enhancements
Audio: Fixed detection of duplex audio devices on macOS (5.0 regression; bug #20171)
Network: Fixed link status reporting for "not attached" adapters
Network: Fixed connectivity issues with e1000 in OS/2 guests (6.1.18 regression; bug #20148)
Network: Fixed VxWorks e1000 driver compatibility issue (bug #20182)
Network: Fixed GUI checks for port forwarding rules rejecting IPv6 with "Nat Network" (bug #14847)
DHCP: Don't crash in the presence of fixed address assignments (bug #20128)
Serial: Fixed possible VM hang when using the a serial port in disconnected mode (bug #19854)
Webcam: Fixed interoperability with v4l2loopback and fixed a crash under certain circumstances (bug #20176)
NVMe: Fixed sporadic Windows VM hang or reboot on high CPU load
VBoxManage: Allow changing network adapter attachment of a saved VM with "modifyvm"
vboximg-mount: Fix for argument processing to honor the '--root' option (6.0 regression; bug #20073)
Linux host and guest: Support kernel versions 5.11 (bug #20198) and 5.12
Linux host: Maximum MTU size increased to 16110 for host-only adapters on Linux kernels 4.10+ (bug #19122)
Linux Guest Additions: Fix vboxvideo module compilation for kernel version 5.10.x
Linux Guest Additions: Fixed kernel module build for RHEL 8.4 beta and CentOS Stream (bug #20289)

File "fixes-for-5.11.patch" is deleted. The issue is fixed upstream.
File "vboxautostart.sh" is replaced by "vboxautostart-service.sh"
File "vboxautostart.service" is replaced by "vboxautostart-service.service"
Fixes boo#1183329 "virtualbox 6.1.18 crashes when it runs nested VM"
Fixes boo#1183125 "Leap 15.3 installation in Virtualbox without VBox integration"
Fixes CVE-2021-2264 and boo#1184542. The directory for the .start files for
autostarting VMs is moved from /etc/vbox to /etc/vbox/autostart.d. In addition, the autostart
service is hardened (by Oracle).