We might get a call to get_memory_region() before the device has been
realized. We should return a consistent value, as the return value
will e.g. later on be used in the pre_plug handler.

To avoid duplicating too much code, factor the initialization and checks
out into a helper function.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180619134141.29478-12-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>