xen/balloon: decorate PV-only parts with #ifdef CONFIG_XEN_PV

Balloon driver uses several PV-only concepts (xen_start_info,
xen_extra_mem,..) and it seems the simpliest solution to make HVM-only
build happy is to decorate these parts with #ifdefs.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>