src: Unify virObject member name

src: Unify virObject member name

Whenever we declare a new object the first member of the struct
has to be virObject (or any other member of that family). Now, up
until now we did not care about the name of the struct member.
But lets unify it so that we can do some checks at compile time
later.

The unified name is 'parent'.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>