powerpc/perf/hv-24x7: Break up single_24x7_request

Break up the function single_24x7_request() into smaller functions.
This would later enable us to "prepare" a multi-event request
buffer and then submit a single hcall for several events.

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