When the transmit FIFO is emptied or enabled, the transmitter
hold register is empty.  When it is disabled, it is also emptied and
in addition the previous contents of the transmitter hold register
are discarded.  In either case, the THRE bit in LSR must be set and
THRI raised.

When the receive FIFO is emptied or enabled, the data ready and break
bits must be cleared in LSR.  Likewise when the receive FIFO is disabled.
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>