xen/x86: allow "earlyprintk=xen" to work for PV Dom0

With preferred consoles "tty" and "hvc" announced as preferred,
registering "xenboot" early won't result in use of the console: It also
needs to be registered as preferred. Generalize this from being DomU-
only so far.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/d4a34540-a476-df2c-bca6-732d0d58c5f0@suse.com


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