console/serial: bump buffer from 16K to 32K

Testing on a Kaby Lake box with 8 CPUs leads to the serial buffer
being filled halfway during dom0 boot, and thus a non-trivial chunk of
Linux boot messages are dropped.

Increasing the buffer to 32K does fix the issue and Linux boot
messages are no longer dropped.  There's no justification either on
why 16K was chosen, and hence bumping to 32K in order to cope with
current systems generating output faster does seem appropriate to have
a better user experience with the provided defaults.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>

xen/drivers/char/Kconfig

@@ -77,13 +77,13 @@ config HAS_EHCI
 
 config SERIAL_TX_BUFSIZE
 	int "Size of the transmit serial buffer"
-	default 16384
+	default 32768
 	help
 	  Controls the default size of the transmit buffer (in bytes) used by
 	  the serial driver.  Note the value provided will be rounded down to
 	  the nearest power of 2.
 
-	  Default value is 16384 (16kiB).
+	  Default value is 32768 (32KiB).
 
 config XHCI
 	bool "XHCI DbC UART driver"