xen: add raw console write functions for debug

Add a couple of functions which can write directly to the Xen console
for debugging.  This output ends up on the host's dom0 console
(assuming it allows the domain to write there).

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>