BHYVE: OS-7215 bhyve uart broken for win2016 guest

Reviewed by: Joshua M. Clulow <jmc@joyent.com>
Reviewed by: Hans Rosenfeld <hans.rosenfeld@joyent.com>
Approved by: Hans Rosenfeld <hans.rosenfeld@joyent.com>

usr/src/cmd/bhyve/uart_emul.c

@@ -519,6 +519,16 @@ uart_write(struct uart_softc *sc, int offset, uint8_t value)
 		sc->thre_int_pending = true;
 		break;
 	case REG_IER:
+#ifndef	__FreeBSD__
+		/*
+		 * Assert an interrupt if re-enabling the THRE intr, since we
+		 * always report THRE as active in the status register.
+		 */
+		if ((sc->ier & IER_ETXRDY) == 0 &&
+		    (value & IER_ETXRDY) != 0) {
+			sc->thre_int_pending = true;
+		}
+#endif
 		/*
 		 * Apply mask so that bits 4-7 are 0
 		 * Also enables bits 0-3 only if they're 1