Split out struct kvmppc_vcore creation to separate function

No code changes, just split it out to a function so that with the addition
of micro partition prefetch buffer allocation (in subsequent patch) looks
neater and doesn't require excessive indentation.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>