If a bhyve UEFI-booted guest is launched with a com1,stdio console, that console is useful as a system console on the VM, and in the case of Windows Server, the SAC management console.
Unfortunately, the com1,stdio console receives various error reporting from bhyve, including keyboard/mouse and "rdmsr to register" notifications.
Workaround: Use an nmdm or pty console device
Desirable solution: Proper bhyve logging to a mechanism such as one or more log files or syslog.
I'm not sure if this is a bug. Bhyve, as a usual utility, displays the stderr on the terminal. Therefore, you can simply redirect stderr output to a file.
# bhyve ... -l com1,stdio ... 2>vmerr.log
# cat vmerr.log
rdmsr to register 0x4e on vcpu 0
rdmsr to register 0x34 on vcpu 0
wrmsr to register 0xc8f(0) on vcpu 0
Unhandled ps2 mouse command 0xe1
Unhandled ps2 mouse command 0x88
Unhandled ps2 keyboard command 0xf6
That workaround is preferable to every suggestion to date.
I don't know if grehan@ requested a ticket for another reason but I will mark this as resolved.