cpu-common.c: export cpu_get_free_index to be reused later

cpu-common.c: export cpu_get_free_index to be reused later

This helper provides an easy way to identify the next available free cpu
index which can be used for vcpu creation. Until now, this is being
called at a very later stage and there is a need to be able to call it
earlier (for now, with ppc64) hence the need to export.

Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>