bhyve uart broken for win2016 guest

[pmooney_pfmooney.com]

Summary

When running Win2016 guests under bhyve, data transfers over serial ports from the guest to the host would stall. The investigation recorded in OS-7215 found that the UART emulation was not properly asserting an interrupt when certain register values were manipulated from the guest. This would cause the outgoing transmission to stall while the guest waited for a TX interrupt that would never arrive.

Test Plan

With this patch in place, verify that transfers into and out-of Windows and non-Windows guests flow as expected. This fix has been in SmartOS bhyve since September 2018.

[rgrimes]

I removed a redundant "to" in the description. Patch looks good to me. Copyright over to jhb@

[pmooney_pfmooney.com]

This issue was addressed when it was effectively side-pulled in commit #345158, despite having been posted here and accepted a week prior.