PV console doesn't require Xen to be running as a shim and only
requires CONFIG_XEN_GUEST and running as a Xen guest. Update the
documentation accordingly.
Fixes: 4f6609d6a6 ("x86/guest: use PV console for Xen/Dom0 I/O")
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>