dimm: Convert to DEFINE_PROP_LINK

Unlike the usual object_property_add_link() invocations in other
devices, dimm checks the "is mapped" state of the backend in addition to
qdev_prop_allow_set_link_before_realize. To convert it without
specializing DEFINE_PROP_LINK which always uses the qdev general check
callback, move the extra check to device realize time.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170714021509.23681-11-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
