VirtualBox 6.1.8 Build 137981

What's new in this version:

- GUI: Fix several layout and mouse position handling bugs with soft keyboard
- GUI: Fixed crash on last VM removed
- GUI and API: Allow renaming VMs which are in saved state
- Serial: Fixed slow guest output when using the TCP server mode without anyone being connected
- Guest Additions: Restored 'VBoxClient--checkhostversion' functionality
- Guest Additions: Fixed resizing and multi monitor handling for X11 guests
- Guest Additions: Build problems fix with Oracle Linux 8.2 (Red Hat compatible kernel) / Red Hat Enterprise Linux 8.2 / CentOS 8.2
- Guest Control/VBoxManage: Fixed handling of multiple environment variables supplied to 'VBoxManage guestcontrol VM run'
- Guest Control: Implemented support for long(er) command lines
- Guest Control: Various stability improvements