xen/x86: make "earlyprintk=xen" work for HVM/PVH DomU

xenboot_write_console() is dealing with these quite fine so I don't see
why xenboot_console_setup() would return -ENOENT in this case.

Adjust documentation accordingly.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

Link: https://lore.kernel.org/r/3d212583-700e-8b2d-727a-845ef33ac265@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>