kvm: powerpc: book3s hv: Fix vcore leak

add kvmppc_free_vcores() to free the kvmppc_vcore structures
that we allocate for a guest, which are currently being leaked.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>