Console: introduce console=none command line parameter

Console: introduce console=none command line parameter

Currenty, not specifying 'console=<foo>' on the command line causes
Xen to default to 'vga'.  Alternativly, the user can explicitly
specifiy 'console=vga|com1|com2'.

However, there is no way to specify that neither vga nor serial should
be used.  Specifying 'console=' does have the effect that neither vga
nor serial is set up, but at the cost of an "Bad console= option ''"
warning.

Therefore, expliticly support a 'console=none' option which does not
set up vga and does not set up serial, but does not trigger the bad
console warning.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>