xen: ifdef CONFIG_HIBERNATE_CALLBACKS xen_*_suspend

xen_hvm_post_suspend, xen_pre_suspend, xen_post_suspend are only used if
CONFIG_HIBERNATE_CALLBACKS is defined, resulting in:

drivers/xen/manage.c:46:13: warning: ‘xen_hvm_post_suspend’ defined but not used [-Wunused-function]
drivers/xen/manage.c:52:13: warning: ‘xen_pre_suspend’ defined but not used [-Wunused-function]
drivers/xen/manage.c:59:13: warning: ‘xen_post_suspend’ defined but not used [-Wunused-function]

If the kernel config is missing CONFIG_HIBERNATE_CALLBACKS.

Simply ifdef CONFIG_HIBERNATE_CALLBACKS the three functions.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>