KVM: PPC: Do not create debugfs if fail to create vcpu

If fail to create the vcpu, we should not create the debugfs
for it.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Alexander Graf <agraf@suse.de>
Cc: stable@kernel.org
Signed-off-by: Avi Kivity <avi@redhat.com>