stubs: Add vm state change handler stubs

They will be used by BlockBackend code in block-obj-y, which doesn't
always get linked with common-obj-y. Add stubs to keep ld happy.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170815130740.31229-2-famz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>