doc, xen: Mention 'earlyprintk=xen' in the documentation.

commit 2482a92e7d17187301d7313cfe5021b13393a0b4 upstream.

The earlyprintk for Xen PV guests utilizes a simple hypercall
(console_io) to provide output to Xen emergency console.

Note that the Xen hypervisor should be booted with 'loglevel=all'
to output said information.

Reported-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Link: http://lkml.kernel.org/r/1361825650-14031-2-git-send-email-konrad.wilk@oracle.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>