stubs: Add stubs for ram block API

These functions will be wanted by block-obj-y but the actual definition
is in obj-y, so stub them to keep the linker happy.

Signed-off-by: Fam Zheng <famz@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20180110091846.10699-2-famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>