AMD bhyve fails linux unless console is stdout.

[Zaphod Beeblebrox]

I have an AMD "hammer" (9590) running a bhyve with a VM that used to be a
"droplet".  All is good if I run "vm -f start <droplet>" it works.  That
is: the console is attached to stdin/stdout of the vm command.

If, however, I start without the -f, I get the linux kernel "not" booting
with output of:

[   54.548000] serial8250: too much work for irq4
[   57.412000] serial8250: too much work for irq4
[   61.040000] [sched_delayed] sched: RT throttling activated
[   61.044000] serial8250: too much work for irq4
[   63.892000] serial8250: too much work for irq4
[   66.760000] serial8250: too much work for irq4
[   69.656000] serial8250: too much work for irq4

... so suboptimal.