spapr: rename spapr_vcpu_id() to spapr_get_vcpu_id()

spapr: rename spapr_vcpu_id() to spapr_get_vcpu_id()

The spapr_vcpu_id() function is an accessor actually. Let's rename it
for symmetry with the recently added spapr_set_vcpu_id() helper.

The motivation behind this is that a later patch will consolidate
the VCPU id formula in a function and spapr_vcpu_id looks like an
appropriate name.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit 14bb4486c819ea797a151b3e0fe53d6f5c7b3fc5)
Signed-off-by: Greg Kurz <groug@kaod.org>