xen: fix "xen: implement save/restore"

-tip testing found the following build breakage:

  drivers/built-in.o: In function `xen_suspend':
  manage.c:(.text+0x4390f): undefined reference to `xen_console_resume'

with this config:

  http://redhat.com/~mingo/misc/config-Thu_May_29_09_23_16_CEST_2008.bad



i have bisected it down to:

|  commit 0e91398f
|  Author: Jeremy Fitzhardinge <jeremy@goop.org>
|  Date:   Mon May 26 23:31:27 2008 +0100
|
|      xen: implement save/restore

the problem is that drivers/xen/manage.c is built unconditionally if
CONFIG_XEN is enabled and makes use of xen_suspend(), but
drivers/char/hvc_xen.c, where the xen_suspend() method is implemented,
is only build if CONFIG_HVC_XEN=y as well.

i have solved this by providing a NOP implementation for xen_suspend()
in the !CONFIG_HVC_XEN case.

Signed-off-by: Ingo Molnar <mingo@elte.hu>