qom: Make all interface types abstract

"qom-list-types abstract=false" currently returns all interface
types, as if they were not abstract. Fix this by making sure all
interface types are abstract.

All interface types have instance_size == 0, so we can use
it to set abstract=true on type_initialize().

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1481567461-2341-1-git-send-email-ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>