xen: make early console also write to debug console

When using "earlyprintk=xen", also write the console output to the raw
debug console.  This will appear on dom0's console if the hypervisor
has been compiled to allow it.

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