qom: Do not register interface "types" in the type table and fix names

There should be no need to look up nor enumerate the interface "types",
whose "classes" are really just vtables.  Just create the types and
add them to the interface list of the parent type.

Interfaces not registering their type anymore means that accessing
superclass::interface by type name will fail when initializing
subclass::interface.  Thus, we need to pre-initialize the subclass's
parent_type field before calling type_initialize.  Apart from this, the
interface "types" should never be used and thus it is harmless to leave
them out of the hashtable.

Further, the interface types had a bug with interfaces that are
inherited from a superclass:  The implementation type name was wrong
(for example it was subclass::superclass::interface rather than
just subclass::interface).  This patch fixes this as well.

Reported-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
