spapr_vscsi: fix build error introduced by f19661c8

A typo introduced in f19661c8 prevents qemu from building when configured
with --enable-trace-backend=dtrace.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>