ivshmem: fix memory backend leak

ivshmem: fix memory backend leak

object_new() returns a new backend with refcount == 1 and
then later object_property_add_child() increases refcount to 2
So when ivshmem is destroyed, the backend it has created isn't
destroyed along with it as children cleanup will bring
backend's refcount only to 1, which leaks backend including
resources it is using.

Drop the original reference from object_new() once backend
is attached to its parent.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1541069086-167036-1-git-send-email-imammedo@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Fixes: 5503e285041979dd29698ecb41729b3b22622e8d
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>