debian/control: Add missing Replaces on old xen-utils-common

Previously the xenstore utility manpages were erroneously in
xen-utils-common.  We need to declare Replaces so that dpkg lets us
take them over rather than regarding it as a file conflict.

I think we can safely drop the old Conflicts/Replaces from Xen 3.1.0
days.

Signed-off-by: Ian Jackson <ian.jackson@citrix.com>