powerpc/perf/hv-24x7: Modify definition of request and result buffers

The parameters to the 24x7 HCALL have variable number of elements in them.
Set the minimum number of such elements to 1 rather than 0 and eliminate
the temporary structures.

This would enable us to submit multiple counter requests and process
multiple results from a single HCALL (in a follow on patch).

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>