On x86, Xen does not claim any serial port unless the user

On x86, Xen does not claim any serial port unless the user
explicitly specifies a 'com1=' or 'com2=' configuration
string. This prevents users from being surprised when Xen
stops domain0 from accessing serial port registers.

On ia64 we still allow serial ports to get automatically
configured and locked down even if no 'com1' is specified
on the command line. This maintains the default preferred
by the IA64 developers.

Also, the method for specifying 'use existing baud rate'
has changed. Instead of specifying com1=0, you must now
specify e.g., com1=auto

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