Now that we have a bit more control, let's convert our object into
a lockable object and let that magic handle the create and lock/unlock.

This commit also introduces virInterfaceObjEndAPI in order to handle the
lock unlock and object unref in one call for consumers returning a NULL
obj upon return. This removes the need for virInterfaceObj{Lock|Unlock}
external API's.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>