qom: do not return root for empty path

An empty path will return the sole object of that type in the
QOM tree.  This is different from "/", which returns the root.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1366303444-24620-7-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>