Reduce spin-waiting in Xen serial driver:

Reduce spin-waiting in Xen serial driver:
 1. Split the serial port lock into receiver and transmitter locks.
 2. In the ns16550 interrupt, only call the generic serial service
    routines if there is receive (or transmit) work to do.
 3. In the generic transmit ISR, avoid long spin-waits by *trying*
    to take the transmitter lock and, if that fails, check again
    whether the transmitter is empty. This will allow us to bail
    bail quickly if there is a long-term lock holder stuffing lots
    of bytes.

Also, gdbstub should be setting its serial handle in synchronous mode,
just for sanity.

Signed-off-by: Keir Fraser <keir@xensource.com>